Tactically, however, if you want a SAM division operating independently, it is crazy talk. What would be the purpose of such a formation? It has no tanks or infantry, so it can't take or hold ground (it can't even defend itself). It's equipped with relatively short-range systems, so it can't defend a big area against air attack, and it isn't integrated with other formations so it has nothing except itself to defend in any case. It seems like a solution in search of a problem.
Small area? If the unit is composed of mix of, say SA-11/Kubs and SA-12/S-300V and closer range SAM's for self defence, it could take care of defense of a fairly large area.
Independently? No unit operates truly independently, and for some tasks infantry / marine / parachute / allied units could be tasked to self-defense duties
I would see several possible tasks for such formation for, say, Soviets in 1980's:
1.) Help for friendly governments globally without putting Soviet personnel on direct front line - granted SAM units may suffer casualties too but they would be deniable
2.) Reinforcing Army and Navy operations at critical stages - protecting key transportation during mobilization, support of key offensives, support of coastal defenses and amphibious operations - such as going to Denmark from East Germany, for example
3.) Complicating enemy air operations by mobile operations - deployment of unit close to front line or improbable location at some point to eliminate enemy support aircraft, for example, complicating enemy planning for air operations as the mobile SAM's would be an unexpected element
4.) Help for defense of critical homeland installations and bases at important phases of operations
Why independent? If a GBAD unit is tasked to support ground defense or to defend a key area it's key task is to defeat or hinder enemy air defense. Independent unit can be tasked to destroy enemy aircraft and could design it's operations differently, if decided. For example, when defending an area GBAD is based to effect enemy ingress route, when if one wants to cause casualties it might well be better to be based on to effect enemy egress route.