Pazuzu's Wikipedia article reveals his motivations.
The big picture is in the goals section:
Pazuzu is viewed by the denizens of the Abyss with resentment and grudging respect. Unlike most demon lords, he is not interested in conquest of the Abyss. His main concern is winning followers on the Prime Material Plane by tempting or deceiving those of good alignment. He aids any good creature who summons him, in order to tempt him or her to evil. In the Monster Manual II, he is said to be on good terms with daemons (now known as yugoloths) and the dukes of Hell.
So, he grants wishes to tempt good-aligned people into doing bad things. Simple enough. But how does it work?
The details are in the Cult of Pazuzu section:
Cults of Pazuzu are started when someone in desperate need discovers Pazuzu's name and pleads for his help. When someone utters the name "Pazuzu" three consecutive times, the demon lord is telepathically linked to that person for one minute. During that time, Pazuzu is capable of reading the thoughts of whoever he is linked to, in addition to checking whether they are lawful or good.
After confirming that whoever said his name isn't endeavoring to capture him on the Material Plane, Pazuzu appears before the speaker. Pazuzu grants the person aid, usually via his wish spell-like ability, and in reply asks that his name be repeated to others.
The mortal's miraculous turn of fortune attracts other desperate souls. Pazuzu continues to grant aid when called upon, each time producing more evil results. Eventually the cultists come to depend on him for their success, at which point he reveals his true nature. Those who try to repent are tortured and sacrificed.
After a cult has committed its first sacrifice, Pazuzu abandons it and seeks out other prospects. Eventually the cult is destroyed by lawful and good forces; looters discover Pazuzu's name and the cycle's continuance is assured.
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This is essentially the way the scam works:
Pazuzu appears to a good character, and grants them a wish. The wish is entirely above-the-board, with the only stipulation being that the character promises to spread Pazuzu's name.
While this isn't a binding contract, note that Pazuzu has already read the subject's mind. He won't appear if his scam won't work. For example, because:
The subject knows Pazuzu is a demon.
The subject fully intends to disregard any conditions on aid granted to him.
The subject knows about the scheme.
The subject intends to harm or trap Pazuzu.
As more people learn about the wishes Pazuzu grants, he subtly makes the outcome of the wishes more and more evil (but still effective).
Eventually, he has a big cult who are dependent on him for success, and have accepted some morally questionable outcomes from their wishes.
He reveals himself to the cult as a demon. He demands that anyone who attempts to repent be tortured and then sacrificed to him.
Pazuzu leaves the now thoroughly evil cult, and goes in search of new victims.
What does Pazuzu get out of this?
Followers on the material plane, in the form of the cult.
The corruption of good characters to evil characters (eventually).