A beanbag can be very helpful for photographers because in many situations it will allow you to replace a tripod, and it reduces camera shake.
Carrying around a full size tripod can be cumbersome, so placing your camera on top of another object to balance it, or to get a different angle (say on the floor, for example) can be one way to steady it. A beanbag will let you do that in many more situations than a tripod. You can buy one, but you can easily make one yourself too.