Gimbal - What am I missing?

I have been kicking around the idea of getting a gimbal head. I have a Canon 6D and a 1D mark 3 that I use with a Sigma 150-600C. What am I missing by not using a gimbal head? I have never used one, so I am asking for advise. Thanks.

It all depends on what you shoot.... like, gimbals are great for birds in flight

If you don't feel confident when your tele is on the ball head, and are unable to sufficiently track wildlife or sports, a gimbal may help a lot.

Confidence comes because it should balance your rig - you won't feel that you'll drop the camera through a moment's inattention.

Tracking because when the rig is balanced, it's just a matter of steering it, without fighting something off-balance.

This all presupposes that you're working on a tripod - at 300mm there's a lot you can do handheld, but working on sticks is certainly important at low shutter speeds or when you have long minutes or hours where the gear gets too heavy.

A gimbal is good for supporting big, heavy glass in situations where holding it for prolonged periods wouldn't be possible or where you need a slow shutter speed because of low light. When a gimbal is properly balanced you can take your hands off the lens/camera and nothing will move, even without tightening anything down. With big glass this is important because it also means you aren't fighting to keep things upright and stable when you are shooting.

The downside to a gimbal (any head really) is that for panning shots you need to move your body around the tripod as you pan, and if your subject is changing altitudes you will find yourself needing to make corresponding movements in the vertical plane as well. If the action is taking place within a fairly well defined and narrow field of view this isn't much of a problem, but in a situation where I may need to pan through something closer to 180° I would much rather handhold if I have the light. The only lens I typically use my gimbal with is my 600 II, and even then it's usually for wildlife in low light. For BiF I'm generally above 1/1000 at a minimum, and I find handholding to be more effective. With the older and much heavier 600 v1 a gimbal would be necessary more often (almost always for most people).