Strictly speaking, tumbling / cleaning isn't
absolutely necessary.
18 years ago, I used to just take a 30 cal brass bore brush to the inside of my 30 cal rifle cases to get any debris out (dirt, cobwebs, stray pebbles, small mammals), wipe any mud and gunk off, lube, size, and load them. My 45 ammo would just get knocked on the table to dislodge any pebbles, and reloaded, period.
They didn't even start selling tumblers until WAY late in the handloading thing. And liquid cleaners followed quite awhile after THAT.
From late 1800's to what, about the 70's? People didn't really do all the case cleaning prep we do now. And guys like my late Uncle Joe shot a LOT of ammo that was just brushed off and loaded up. In his case, in the early 80's, he started tumbling in media to make them shiney again. But shiney brass was the only real result he was after.
The one thing cleaning does is reduce the amount of maintenance cleaning you have to do on your reloading dies. You get less scratched cases from grit accumulating in the die, have to take them apart less to clean out the interior.
When you are cranking out a batch of 1,000 9x19 or 45 ACP off of a progressive, the last thing you need is the distraction of cleaning out your dies (once you detect scratched cases). It interrupts the process. So I clean anything going to the progressive.
(EDIT: To this day, if I'm pressed for time, I'll skip the tumbling entirely. I shot a high power match with 7.62x51 in November that used un-cleaned Nosler casings, and I won)