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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.

Spirits:

• 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

My question is, why would a spirit trade with a human?

Free Trade is mutually beneficial by definition: each party values what it receives more than what it gives up.

Spirit could get more actual spirit power from what the human gives up, or entertainment, or bragging rights.

Spirits do not consume anything, but they need a source of power. Humans trade gives them that power. Even if they do not need power to survive, it can enable them to do more things, travel further, etc.

Spirits need entertainment b/c their immortal lives leave them bored of most things, but they enjoy observing the randomness of human emotions and their reactions to powers and curses from spirit trade.

Finally, even if humans themselves are not entertaining, there is competition between spirits on who can get a more powerful human to trade with them, or to give up the most. Kinda like fishing by humans, where you mainly brag about size of the fish.

Note that while the trade is "forever", I assume it only lasts until the human dies. Which is a very short period of time for immortal spirits.

Edit: if you say that spirit can only trade once per its own infinite lifetime, then either all spirits have traded already, or none are interested in trading.

They might take whatever the human gives up and use it to elevate themselves above a spirit

Collectors.

https://www.independent.co.uk/life-style/motoring/a-look-inside-the-car-collection-of-michael-fux-a7950721.html

Why does this dude have so many cars? He can only drive one at a time. He has them because he can, and he likes them, and they are bragworthy beautiful things, and then when he visits his rich friends he has something to talk about. Maybe he trades cars with them because he likes specific types of cars for his collections.

So too the spirits. They cannot really use the things the humans give them but they collect that kind of stuff. When the spirits get together they pore over their collections, trade, comment, brag and so on.

One other thing a spirit could do is trade something a human gives up with another human. Fireball spirit might find someone who really wanted a sound nights sleep, and exchange for sleeping (which Fireball spirit has in its collection) will give up the ability to whistle through her nose. Fireball spirit is a whistling aficionado and so makes the swap and improves its collection of various whistling powers.

The sacrifice actually carves off a portion of the person't soul or animating spirit. The spirit either consumes these pieces to sustain or enrich itself or it trades them to something that does use them.

Maybe some high level being is creating an art installation: The concept of taste. It wants the ability to taste different things from several different beings (and the same thing since different people may taste salt differently) so it can perfect it's masterpiece.

There could be a demon who hates humanity and is willing to pay for anything that diminishes humanity.

The main thing is that the spirit itself may or may not be able to use those sacrifices but if the portion of the person the sacrifice represents can be traded, it doesn't matter.

Also, if the other being are so different that they can't interact with the story universe, the spirits are going to be the only contact that the people have. Therefore the people will only know what the spirits tell them or what they can speculate (if the spirits don't talk about it).

EDIT: [addition]

If the spirit can truly only trade once (instead of once for each spell), maybe it is the trade that lets it evolve into the next higher form of spirit. Maybe the spirits are the grubs of the spirit world.