The Roboport Wagon is a cargo wagon that carries robots and brings a robot network with it by train. When its train stops at a station, the wagon's robots go to work around it:

Construction robots build ghosts, repair damaged buildings and deconstruct anything marked for removal, using items from the wagon.
Logistic robots fill nearby requester chests from the wagon and empty nearby active provider chests into it.

When the train is ready to leave, it waits for the robots to fly back, then departs. If the station is already covered by a roboport, the wagon lends its robots and items to that network instead, and takes its robots back when it leaves.

The Robot Depot Stop is a special train stop for your base. There, your base robots restock the wagon with the items you've asked for and unload everything else into storage. Robots and repair packs always stay in the wagon.