Bean bags are a very flexible means of support and also protect yout equipment from the rough and dirty surface of gate posts, walls and the like. The bag is effectively a small cushon filled with tiny pellets of polystyrene. I found it usefull to have a range of bags in different sizes and shapes to either support in different ways or different equipment.
The 'small' bag is usefull as a pocketable item to havewithyou at all times when you want to rest the camera on posts/walls. The 'medium' bag is good for telephoto work and supporting the camera on the floor. The 'Large' bag is necessary for the weight of long telephoto lenses. Finally the 'H' bag is specifically designed to straddle the window of a car/van.