To most people the Topping Lift is the rope on the side of the mast that hasn't been touched the entire time they have owned the boat and is currently covered over with algae growth! That mossy green line actually serves many purposes and should be adjusted frequently.
The main purpose of the Topping Lift is to hold the boom up when the sail is not raised. Most people set this line once when the boat is new and never adjust it again. This is why the topping lift is often forgotten for the life of the boat, allowing the line to rot away on the cleat.
The problem with "setting and forgetting" the topping lift is as the sail is raised, the topping lift will become slack and begin to bow out. The topping lift can even wrap around the backstay which can sometimes foul on a backstay adjuster. When the sail is raised, the topping lift should be adjusted to take out any slack in it while taking care to not over-tighten it.
When the mainsail is reefed, the boom will rise to meet the new clew, this will also induce a lot of slack into the topping lift. Once again, it should be adjusted, taking out the slack line.
Lastly, the topping lift can be used to delay reefing by tensioning it a bit to raise the boom. This will induce a twist into the sail, causing the top of the sail to spill air and reduce its efficiency. Releasing the mainsheet will also induce a twist in the main, spilling air, but only when the boom is eased out. Using the topping lift to induce a twist will allow you to spill air while close hauled with the main sheeted in tight.
One last reason the topping lift shouldn't be ignored is this line can serve as a spare halyard for the main. The topping lift doesn't really undergo much stress, and could be rigged with a very small and weak line. If something were to happen to the main halyard, it would be awfully nice to simply hook the topping lift to the head of the mainsail and raise the sail up to continue sailing onward. For this reason, the topping lift should be greatly oversized and made to the same specifications of the main halyard.
Next time you look at your rigging, dust off the topping lift and inspect it with the same scrutiny that you give your other halyards.