You don’t need to have an assist arm and you don’t find them on manual tire changers but they are very useful. It saves a lot of time and effort during the tire change.
It is a pneumatically powered arm that locks in place. Then, it exerts pressure on the walls of the tire but does not damage them. There is a clamp that lets the tire rotate as the turntable moves. The clamp is very efficient at unseating the bead.