Raise the boom with topping lift to give the main more camber in light winds. Lower the boom for less camber to de-power (flatten) the sail in high winds. I believe you can head up better in this config aswell. More experinced sailors please correct me if ive missed something.
The topping lift isn't going to help lower the boom in high winds (although if it's set too tight it will stop you doing that). Flattening the sail is done via vang, cunningham, reef, mainsheet, outhaul et.
GREETINGS EARTHLINGS; You can use the topping lift to scandlise the main (take out the drive ) when closing in onto targets. can also use it to get stuff aboard from the dinghy or quay-side AS ALWAYS GO SAFE