I think that we talked about this ingame, but to recap:
The wallet keeps items that would normally take up bag space; and there isn't a limit to how much it holds.
Such items as festival tokens (flowers & fish as well), taxidermy items, barter items... I think there's more, but not sure what at this point. Some of those are shared across your account, and some are just limited to the alt looking at the wallet at a time. It does show how much gold, silver, copper you have, but that is one of the things that is specific to the person looking at it, as are the festival items. You can see the marks & things that are rewards from the skirmishes there as well... they are across the account.
I think it is one of the best purchases I made from the store for the saving of bag space alone. The convenience of it is a great bonus!