>>538644202
This may sound obvious, but think of them as simultaneous actions - in other words, if you have a sequence which would normally be vulnerable in some way, assists can cover the gap.
Imagine a pressure sequence that ends in a negative on block or even punishable move, if you time an assist correctly they can add active frames where you would otherwise be punished.
You can also use them to cover opponent's options. Are you playing Blitz with Darius in your backline, and your opponent is in grab range? Hit Blitz command grab + Darius AA apprehend at the same time. You either grab them or hit the opponent with apprehend, bringing them back down with defensive options disabled, giving you two mixups in a row.
Obviously you can also use them for combo extensions, but that is less valuable in actual gameplay than 'fill in the gaps' to make your neutral and pressure better.