A roll cage is exactly like the name implies. An internal steel-tube cage built internally in any car to protect occupants from excessive damage from impacts. The Roll cage does not protect occupants from projectile, blast or flame damage.
For instance, two cars made exactly the same with one equipped with a roll cage and the other without can take the same impact, but the one without it will have the occupants incapacitated (knocked out) while the other cars occupants take a small amount of damage but are able to function and control the car.
Roll cages use the bulk available on any chassis. Some chassis use more bulk for roll cages and cost more than others obviously. You must decide if you want to protect your occupants from impact damage or use the bulk for weapons, gunners or ammo instead. Any vehicle that uses a Mounted Ram, Reinforced Ram or Mounted Spikes will benefit from having a Roll Cage installed.
Roll Cages cannot be added or removed in camps. (bug)