Offering something in return is a friendly gesture, the Gods don't demand it and it's not some sort of a "payment". And the summoning page near the end offers examples as to what to give:
JoS site said:
Always offer a token of appreciation and gratitude in return for his/her services. Some ideas are publicity for the Demon, working against the enemy, working to advance Satanism or any service that you both can agree upon.
Daemons love to help those that help themselves. You’ll notice Demons will also do favors a plenty for you if you pay it forward, doing rituals schedules and such without asking for anything. Demons never let good work go unrewarded in my experience.