There is a litany of maintenance from hardware failure e.g. a camera is tampered with and needs service, hardware failure, hard drive, POE switch, Camera, NVR etc. Your company may make a network or router change and the system goes down and needs to get back up and running. User updates on the software side, granting denying access to a user, remote access etc. Much of the software service can be handled internally, but you want to make sure you or someone in the organization is properly trained to make changes and completely understands what changes to the software configuration means to the functionality of the system.
This is just of the top of my head, it is a bit more complex than what I've explained here to maintain and service and IP system.