I believe the part you are referring to is the spirits called "The Mercenary" I don't think there is a concept of "Spirit Pacts" with rules and such. Its all about motives. If a character is desperate or greedy and needs supernatural help, and also knows how to contact spirits, either by medium, occult powers or directly in physical world, he might struck a deal with the spirit.
In the book's example, a spirit with the power to possess mortals could help another human being by possessing his enemies and such. But don't limit yourself with possession. Spirit's can influence the physical world with "Reach" numen. They also can manipulate objects/people/feelings/concepts of their purview. A night club owner could send an anger/murder spirit to his rivals club to cause terror, which ultimately disrupts his rivals business.
There are so many things a mortal could ask from a spirit. but what about the spirits? what do they want? well first of all they need essence to survive, and essence is not always abundant. Spirits regain their lost essence by being in the proximity of their purview. So a love spirit could gain essence just by attending a romantic dinner. mortals can setup the scene for the spirit so the spirit could gain essence. for other spirits, they might have enemies in the physical world, like werewolves which try to kill or mages which try to control them. the mortal could be used by the spirit to eliminate or disable his enemies.
suppose a nature spirit was happy and all in a park, but suddenly, construction vehicles barge in and begin destroying the trees which is the source of essence for the spirit. So the spirit sends message (with reach numen) to a nearby mortal to stop the construction of a new mall on the park. the mortal asks for a magical night with his girlfriend in the park after this issue is resolved. the deal is struck and the mortal organizes environmentalists to protest the construction and see where this goes.