For me, one of the uses of AS is to enter the fray with an initial price I'm more than willing to pay. So I'm in the game but I'm freed from concern, i.e., I don't have to worry that I've bid too much, and I don't have to remember to bid later on.
After my initial bid, I can raise my offer if I decide either that the item is worth more than I initially thought. Example: I buy hobby items and sometimes I'll fall in love with an item after I've given it some thought. Or, if I initially thought I could pick it off for peanuts, an Outbid might persuade me to up the ante. Often there's some combination of those things.
AS also allows for a change of heart - I'm in, but if I decide to withdraw my offer, I am free to do so without dealing with any eBay red tape.