I cannot speak for any given photographer, but I use monopods for two main reasons. Mobility and Stability. The fact that a monopod allows me rest my hands and arms is a bonus. Hand holding is not so great for longer shooting sessions, like a 3 hour ball game.
A monopod is far more mobile than a tripod. It is quick to set up, and quick to move out of the way of violent play moving in your direction.
Your photographs are sharper when you use some type of support. The biggest benefactor of IS in a lens is the AF system in the camera body. Having a stable image makes it easier for the camera’s AF system to recognize subjects and track them. While it is true that IS helps to reduce camera shake when shooting handheld, using a camera support system is even better.
Finally, some of the large primes are pretty heavy. They are too heavy to use handheld for extended periods of time. Not only do you give your hands and arms a rest with a camera support system, you can keep the lens aimed and focused at the subject area between shots.
If you decide to invest in a monopod, I strongly recommend that you find one that is at least as tall as you are. I say this for two reasons. One, monopod and tripod legs are more stable and stronger when you do not fully extend them.
Two, when you have a tilt head mounted on a monopod you can point the camera upward without having to squat at the same time. When you aim the camera upward, the rear panel gets angled toward you. The best position for your body is “under” the camera body, not behind it.
For most sports, aiming the camera upward is a pretty rare shooting scenario. But, it is a common one for photographing wildlife and BIF. Personally, I have found using a gimbal with anything less than a LONG lens to be awkward.
Even with my Sigma 150-600mm, the camera body can swing over the center of the tripod when I angle it upward. The camera body moves slightly down and away from me when I angle upward with a gimbal. A LONG lens keeps the camera body away from the tripod.
I have found a monopod with a quality tilt head to be easier than my gimbal. If you have ever used a ball head, then you should understand the benefit of having friction control. Tilt heads do not have a separate friction control. I use a Kirk MPA-2 tilt head. While it does not have separate friction, the single knob that it has can rotate at least 720 degrees, which allows you dial in precise amounts of friction on the tilt mechanism.
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