It makes it broader-spectrum. Ampicillin has more resistance than the combination in Clavamox. Clavulanic acid makes the combo less likely to encounter resistant bacteria. It does not taste good, and it makes a lot of animals and people sick to their stomach. It should be taken/given with food. Even then, in dogs and cats, it still causes about 10% to puke. If they puke just once, we usually leave them on it. If they puke twice, then we take them off and use something else.
I've never used it in chickens, but guess there is no reason why not. Chickens won't puke, but the nausea could put them off their feed. I would use the same withdrawal time as ampicillin, which is no doubt listed somewhere (I have it bookmarked, but my list is so long I'd have to hunt for it). I think a week sounds right, though.