there is 1 more reason also:
when u reach lvl cap, u normally do not gain any exp after. however, if u have EXP bonus from house, it gets drained when killing mobs, even if u r at lvl cap. this indicates that in a coding manner, u "get" exp, the exp-bonus is drained, and just get trashed after.
in short terms, with that item, u can save EXP house bonus even when u r at cap, making it alot easier to lvl up when new cap comes