First, Galvanic Relay makes you play the exiled cards on your next turn, not the current turn, so youll be slightly less pressed for mana
Sure, copying the right legendary creature just twice might be enough to win you the game, especially if were copying Kozilek But think of the possibilities
For instance, Eldritch Evolution lets me get a creature directly out from my deck
A lot of tapping and stunning leads to frustrating gameplay and so does having everyone fly
If it's legendary, it also gains trample until end of turn
Both have a great board presence as solid trampling bodies that can swing and win, while also having shroud/hexproof to keep your foes from mucking about and slaying it