W'ing (when poison bar is filled) and getting HP back.

It allows you to spam E in split pushes while not losing too much mana (without it you gonna get low on mana after 5 uses of E) and on a split push fizz, being able to use abilities without losing mana and hp , and sheen makes it better.
also after RoA finishes stacking you get 100 AP which isn't a bad deal, you get good mana, good hp, and 100 AP, it will delay your early game powerspike (Lich Bane later) but it will later on be more useful, you can try a RoAless build, i guess it would be good if you're behind and you need your spike items early.