In my world, there are spirits who can "trade" with humans. The trade is done when a spirit swears to gods to be a human's servant for using a specific spell (for example, in case of fireball, the spirit must create the exact same fireball the human demands and send it to the exact destination using its abilities), and in exchange the human accepts to make a specific sacrifice once and for all (for example, this could be "losing the ability to sleep").
Trade isn't done until both of them are fine with the trade.
Spirits can only trade with one person at a time, but they can trade more than once in their lifetime. Until the human dies, they can't trade with anyone else.
- can't be seen by humans, except in dreams
- don't have bodies
- don't need to eat or consume anything
- are immortal
- can speak any language, and can enter any dream
- know only 1 specific spell
- are a separate race
My question is, why would a spirit trade with a human?