Wow I never expected all of those things to be separate stations. The sizing and seating primer has always been one station in.my mind. I never expected to see three powder stations. Is there advantages to separating out all these steps. Really asking because I'm not a progressive guy not just trolling.
This is going to be long, but I hope you will take the time to read it.
In a press where you are feeding bullets by hand and the shell plate doesn't rotate, some of the parts aren't needed. Realistically, you do most of these steps, though. For example, you trim your rifle brass, right? You swage or team the primer pocket on rifle brass with crimped primer pockets, right? Of course you do(or you should, anyway). You seat a primer, right?
Let's break it down into the stages/stations, and compare that between the two.
Case feeding: This has to be done somewhere, or else the brass is never making it to the press. With a single stage or turret press, you are typically the case feeder, and you feed the brass into the shell holder by hand. On a progressive, some of them are fed by hand, and some are fed by an actual tool for feeding cases. In the case of the mark 7(and Dillon and Hornady progressives with a case feeder), there is a hopper where the brass is dumped in. It orient's them the correct direction, and drops them down a tube where they are fed into the shell plate. So, this would be a station that is typically done by hand with a single stage or turret press. Most people don't give any thought to this, but it is effectively a "stage" that they are doing on their own. With a progressive, this stage is typically just given its own station, but sometimes integrated with the first station(sizing/decapping/etc).
Resize/decap: One way or another, we all knock the spent primer out of our brass. Some people do it with the standard resizing die, others do it with a universal decapper, and then resize later. I prefer using a universal decapper, as it gives me a more positive removal of the primer(mine has a spring that shoots the primer down once it is free of the pocket, which prevents it from getting sucked back up into the brass and binding things up). If you do this off-press, it is still another stage, even though most people don't think of it as being one.
Resize/trim/swage: Let's start with the resizing portion. You may have done that in the decapping step, you may have not. Either way, it has to happen at some point. I use a Lyman trim die that resizes to the proper size, while also preparing for trimming. If you reload rifle brass, you know you have to trim it from time to time. Most people will do this off-press, which means they effectively have another stage that they aren't considering, simply because they do it off the press. The trim die I use, in addition to resizing, allows a portion of the neck to get squeezed out the top, and then a router with a cutting bit on it trims it to the proper size all in one step. Now, onto swaging. If you reload military brass, you WILL run into crimped primers. You can either ignore the crimp(and likely smash numerous primers by doing so) and have minimal luck priming the brass, ream out the crimp, or swage the crimp out. Most people will go with a reamer, or sometimes a bench mounted swager. Either way, this is another step or stage that has to be completed, which people again don't think much of because it typically isn't being done on-press.
Primer seating: So, if you want your ammo to go bang, they need a primer. Some people will either use a hand primer, and other will use a press mounted priming tool that inserts the primer as the ram is going down. Single stage and turret presses effectively make one station into two stages by doing this at the bottom of the ram's stroke. On a progressive, this is often done in its own station.
Neck expansion: So here is where things get a little weird. Reloading rifle rounds on a single stage or turret press, where you are feeding the bullets by hand and the shell holder doesn't move side to side, it isn't really needed. However, when you are on a progressive and use a bullet feeder, it is helpful for seating boat tail bullets, and even far more helpful for flat based bullets. If you have ever seated flat based bullets by hand, you know you basically have to hold them there until they are in the seating die, or they want to fall over. This isn't really an issue if you are doing it by hand, as you just hold them in place. With a bullet feeder, if they are prone to falling off just by going up and down, you can imagine how quickly they will fall off when the shell plate indexes horizontally to the next station(seating). Using a neck expander opens up the top 1/16 or so of an inch of the case neck just over the bullet diameter, sort of giving it a hole to fall into and stay put. This really has no use for powder charging in rifle rounds.
Powder charge: Powder has to get in there somehow. You can either use a powder thrower off press, powder thrower on press, dippers and a funnel off press, or weigh each charge in a pan and pour it through a funnel off press. Either way, we all do this stage/station at some point. In a progressive, it is done on press, and throws the amount of powder you have adjusted for right into the case.
Powder check: Here is another odd one more specific to progressives. With a single stage or turret, the brass is right there in front of you and the powder charge is clearly visible. On a progressive, the powder throw station can be all the way at the back of the press, and seeing the powder level in the brass can be a challenge without removing the case. Removing the case defeats the idea of the progressive press. A powder check will have a lightweight rod that dips into the case, sits on the powder, and goes up to a certain level when the powder is between a certain charge range. If it isn't in the right range(too low meaning not enough powder or too high meaning too much powder), the powder check will beep or make some sort of screeching noise telling you to stop. This simply alleviates the need to manually look in each case for the proper powder charge.
Bullet feeder: Exactly what the name says. It is a die that feeds bullets and drops them onto the case to be seated instead of you having to do so by hand. Here is there that neck expander for rifles comes in handy. Since the shell plate is what rotates between stations, if the shell plate moves too quickly, the bullet can fall off when the shell plate rotates to the next station. With the neck expanded a tiny bit, it gives the bullet somewhere to firmly stay in place.
Seating: Same as seating on any other press. I prefer crimping in a separate die after seating, but this is all preference.
Crimping: This can sometimes be combined with seating, but I prefer using a lee factory crimp die in a separate station. The crimp removes any excess flare from that neck expanding die as well as crimps it to your preferred amount of crimp. From here, as the shell plate rotates, the base of the brass hits an ejector and gets spit out of the shell plate. On my press, I use a 3d printed funnel that hooks to a shop vac hose, and this drops it right down into an ammo can to be stored away.
Hopefully that provides a little more insight.
