It is rare that there is no need for power within a marquee.
Unless your event will be finished by the time the sun goes down, you’re going to need lighting internally and/or externally. The more lighting you need, the bigger your power requirement.
If you have a DJ for your event, they will definitely need a power supply. Running a presentation or showing a film will need power for both the computer and the projector.
Whether food is being prepared or drinks are being chilled, power is needed.