Unit repairs, base expansion
In a similar fashion, naval units could be repaired at the Naval shipyard, although the Amphibious transport could be repaired at the Service Depot. In both cases, repairs would stop if the commander runs out of resources.
The service depot was considerably more important to the Soviets than the Allies. In the absence of a Tech Outpost or Tech Machine Shop, it was the only way Soviet battlefield commanders can repair vehicles (unless the vehicles are self-repairing, such as elite units and Apocalypse tanks). The Allies could use the mobile Engineer IFV to repair their ground units at no extra cost to the commander; these repairs included the removal of Terror drones.
In Command & Conquer: Red Alert 2 but not in Yuri's Revenge there was a glitch where any vehicle (except for the Amphibious Transport) that stopped on the Service Depot but wasn't given the order to enter the depot for repairs was instantly destroyed.