Nahiri, Forged in Fury

Playing the exiled card follows the normal rules for playing that card. Unless you're casting an Equipment spell this way, you must pay the spell's costs, and you must follow all applicable timing rules. For example, if the card is a creature card, you can cast that card by paying its mana cost only during your main phase while the stack is empty.

You may play the card exiled with Nahiri's ability even if Nahiri or the equipped creature leaves the battlefield or you lose control of them.

One reason you might want to pay its mana cost is if there's an X generic mana in its mana cost. If you cast such a spell without paying its mana cost, you must choose 0 as the value of X. (If you pay its mana cost, you may choose the value of X as normal.)