Mana Vault

so my interest is in [[mana vault]]. what makes it a game changer, my playgroup don't use it so I've never seen it in action. for me you get it out turn 1 and tap it for 3 mana which I understand but then it stays tapped for say turn 2 and turn 3. if you ramp out with a sol ring yeah that helps but having to sacrifice 4 mana every time to untap it seems expensive. are there certain cards you should play with this card, what ways do you exactly abuse this card to make it a game changer. even coming out late game I can't see how it would benefit.