Mobile Sensor Arrays were used by GDI and Nod in the Second Tiberium War. Their primary purpose was detecting cloaked and subterranean units and buildings as well as approaching enemy units. In order to be used the MSA had to be deployed. This was necessary because the highly sensitive microphones and scanner arrays were otherwise distracted by the vehicles' own vibration and motor noise.
GDI commanders used these units to early detect incoming Nod surprise attacks. Also it was valuable for assaults on cloaked bases. Nod commanders on the other hand used the MSA to detect either enemy Nod commanders, mostly during the Nod Civil War. In addition to that, Limpet Drones could be uncovered and destroyed to prevent infiltration.
Eventually they were replaced with more modern sensors, based on lighter, combat-worthy scouting units.