For a very simple piece of photography gear, a monopod is very useful in many different ways. Traditionally, a monopod is used by sports and wildlife photographers who shoot with large, heavy telephoto lenses. Using a monopod with such a setup it suddenly becomes weightless, something you can balance in place with one finger. If you are shooting with a lens that has a lens collar (see above photo, which allows one to attach the monopod to the lens not the camera body) then using a monopod is highly recommended for prolonged shooting. If your lens does not have a lens collar available, but the weight of holding even a non-f/2.8 telephoto lens or sub-500mm lens is taxing, then a monopod will help with that also.
Other ways you can make use of a monopod even without a telephoto lens? You can shoot from a very high perspective by elevating the camera way up in the air holding the monopod at its bottom most point. You can see photographers trying to get shots of someone surrounded by a group of people using this technique.
Also, if you are shooting in a low light situation, but there is not space for the large footprint of a tripod, then a monopod will add a lot of stability helping reduce camera shake, especially in the vertical axis.
How do you use your monopod?