Maintenance units can be used to repair damaged vehicles (even damage to LRF, GPS etc which could not be fixed by the vehicle's crew themselves) or to tow vehicles of equal or lesser weight (rev. 4.000). Note that some damage (e.g. tracks) will be repaired faster if the damaged vehicle's engine has been turned off. Repairs can be accomplished by moving close (~75m) to the unit which requires aid and then waiting. Repairs will automatically commence within a couple of minutes.
Note that damage applied by the mission designer cannot be repaired by using Maintenance vehicles! Instead, the mission designer will have to create a "repair if" condition at design time.