The forward assist is a fail safe for the bcg. When a seizure is caused by dirt build up in the cavity where the bolt carrier group resides, the forward assist, when pressed, pushes at little notches carved in the bolt carrier to push it forward enough to be in battery. From this position firing the rifle again should help the bolt maneuver as normal. The forward assist again, is made for instances of intense combat, where field stripping the rifle and cleaning it are not possible, as well as in environments where getting the rifle dirty cannot be overlooked.

The AR style rifle of the Vietnam war, the M-16 was meant to be the rifle that didn't need to be cleaned. The dust cover, forward assist, and the relatively tight build makes it possible.

Are they useful today?

Well, civilian AR use does not really entail much combat action and firing happens usually in controlled places. So the necessity for the dust cover and the forward assist have gone down. On many of the AR-10 platforms even, the forward assist on the does not actually work. So what is the modern purpose of these two Cold War features? The dust cover may still be able to block some dirt and muck from sliding in to the ejection port, but 9mm platforms don’t even come with the hinges to attach a dust cover.

While the dust cover and forward assist do still pose some level of functionality, they are meant for extreme environments where getting dirty is almost a certainty, and for times where nothing other than that feature will ensure its operation until it is able to be properly maintained.

That is the reason there isn’t really an aftermarket for these pieces. Not only is there not a need to improve upon them, but the standard spec of the features already doesn’t get much use out of it even from the civilians who use them the most.