The charging handle is not attached to the bolt or carrier group. When you pull it rearwards the forward portion of the charging handle engages. However once you lock the bolt back if you push forward on the charging handle there is no resistance. It just slides freely.
Well the same thing would happen if the bolt was only back a hair as well. The charging handle would be engaging nothing on its forward travel.
If the charging handle was attached to the bolt then it would cycle with the bolt, and with its location that would be very bad.
The forward assist essentially compensates for this. Since you cannot operate the charging handle in the opposite direction or move it back and forth to clear anything, meaning you have no way to put forward pressure on the bolt besides pulling it back and letting it go again relying on spring pressure, you get a little button that does it.
When the chamber is dirty enough sometimes it won't go all the way into battery, at which time a forward assist could be useful.
However if it is really that dirty that letting the spring slam it forward won't overcome it, then there is a good chance it won't cycle either, making the forward assist just something to help you with a single shot rifle.
The other thing a forward assist is useful for, and really more likely in a well maintained rifle, is silently chambering ammunition. If you ease the charging handle forward slowly to chamber a round the effort required to strip a round from a full magazine under spring pressure, chamber it, and turn and lock the lugs, and overcome any fouling, may be too much and the action may not close all the way without the momentum of the bolt flying forward from its rearward position. At which point with a forward assist you simply push it all the way home gently. Viola other than the noise of the spring in the buffer tube (which you can reduce in other ways) you can eliminate nearly all other noise with good technique.
While without a forward assist you need to try and push forward on the bolt itself through the ejection port, or pull back on the charging handle and try again, potentially wasting a round or getting a double feed, or yank it back ejecting the partially loaded round and then let it fly forward and give up your stealthy loading.
I like weapons that at least provide a means to deploy silently without clicks, snaps, and pops and other obvious mechanical noises.
You may never use that method, but still have it available.