Opens up the upper body to improve breathing. Improve leverage on climbing. L shaped bar ends can shield your hands from hitting branches. More hand positions.
Short ones can catch the branches.
If improperly installed bar ends can damage you bars. Moves all the equipment mounted on the bar closer in. Bar end might be inserted into the rider's body as the result of a crash. If you have never used bar ends, try them at 45 degrees first. If you find your hands are too far forward, move it up, if it's too far up, move it down.