If you are bouncing out you gear behind a boat and are using a spinning reel you can let the handle rotate and let line out, also if you are bobber fishing you can let line out in a similar manner.
X 2. When trolling, I use the that feature to let my line / gear into the water and count the rotations for depth. When I catch fish at a certain depth (# of rotations), I go back to that same depth each time until I limit out or the fish stop biting (then change the # of rotations / depth until I find the fish again).
When stream fishing, usually w/ bobber and bait, I'll let line out with that feature to cover more water. Sometimes I'll let the bait drift WAY downstream, until it gets into a spot/hole that looks good (and can't get to when bank fishing). I've caught a lot of fish (mostly trout) w/ that trick.