The service-port interface is reserved for out-of-band management of the controller and
system recovery and maintenance in the event of a network failure. It is also the only port that is active when the controller is in boot mode. WiSM use the service port for internal protocol communication between the
controllers and the Supervisor 720. For example, if you want to use "session slot x proc 1" to access WiSM's CLI through sup720, you have to enable a service VLAN which is local to the chassis and is used for
communication between Cisco WiSM and Catalyst Supervisor 720 over a Gigabit Interface on the Supervisor and a service port in the Cisco WiSM. See following link for more detail:
It's not "only" used for intra-chassis communication - or at least, it doesn't have to be. Although no default gateway can be assigned on the service port interface, so it has no way to get off-subnet, you can certainly spread that vlan around as far as you want and communicate directly with the wism from your desk or wherever the service vlan is trunked to.