An ascender is a climbing device that allows you to move upward on a rope. It grips the rope when weight is applied, preventing you from slipping, and can be released when you want to adjust your position or descend. This makes it essential for techniques used in big wall climbing, where you often need to ascend long vertical sections.
Whether you're caving, climbing, canyoneering, routesetting, hauling equipment... you should have an ascender on your harness, period! I use this in conjunction with a rollclip carabiner through the top, a grigri with the break strand redirected through the rollclip, and a foot loop for easy ascending. Personal preference, but I also prefer to use a left-handed ascender even though I'm right-handed.