Their primary purpose is detecting cloaked and subterranean units and buildings, as well as approaching enemy units. In order to be used, the MSA has to be deployed. This is necessary because the highly sensitive microphones and scanner arrays are otherwise distracted by the vehicles' own vibration and motor noise.
Since Nod had monopoly on the two aforementioned technologies, they were only of practical value during the GDI campaign, albeit still available to Slavik's unnamed commander. However, when CABAL went rogue during the Firestorm Crisis, Nod used them extensively against stealth and subterreanean units.
Following the Firestorm Crisis, these units were decommissioned and stealth detection was incorporated into defensive structures and scout units such as the CC-6 Pitbull and Raider buggies.