Meaning of "lose priority" as a cost: You can pay such a cost only once if you are the next player who would receive priority during the current step of the turn. If you have done this, then the only thing you can do when you have priority is to pass; doing this resets the "lose priority". (This prevents you from responding or playing such a spell or ability multiple times before it resolves, unless your opponent responds first in which case you may get another chance.)

When does losing priority ever matter? Especially not for this card.

Why not just make "phase out ~" part of the cost rather than "lose priority", then you have actually something that makes sense.