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 has monopoly on the two aforementioned technologies, GDI forces are the only ones that actually deployed them in the campaign. However, in skirmish/multiplayer matches, the Mobile sensor array is readily available to any GDI or Nod commander.