Gimbal heads are an essential accessory for professional DSLR users who want to shoot moving subjects from a distance. To get detailed closeups without disturbing the scene, they must use super long telephoto lenses, just like the ones you see on wildlife documentarists and sports photographers.
Heavy-duty pro equipment can be extremely bulky, which makes them nearly impossible to hold with just your hands for any length of time. It can even hard to balance a telephoto lens on a normal tripod head, or a ball head. In addition, extreme zoom has the effect of magnifying even tiny hand movements into focus-ruining jumps and shudders.