The topping lift is required exactly why?

[Krumelur]

Every time I notice my topping lift having full tension when the sails are out or swinging around when I forgot to loosen it after putting the sails down, I wonder: what the heck is it needed for anyway?

If there's no tension, the boom isn't coming down. If there's tension, the boom won't come up.
In both cases this is due to the kicker mounted under the boom but also because the boom is mounted on a vertical axle, although the alone would maybe not be able to carry the full weight for a longer time or if someone is doing pull ups.

[symphony2]

Of you have a solid kicker then a topping lift is largely redundant. However many people like to keep it as an alternative for controlling the height of the boom.

[Yngmar]

Good parking spot for the spare halyard, which is what the topping lift really is on our boat

I've seen some boat owners add a piece of bungee cord to it, so that the topping lift can be left slack without, well, being slack and whipping the sail.

[Clivert]

I hoist the boom above head height in harbour and when motoring using the topping lift.
i also use it for going up the mast ( we have an in mast furling main )

[Petef]

Personally, I have slab reefing and use the topping lift, to lift the boom when reefing (my slab reefed sail) prior to letting off the main halyard, and slackening off the vang to lower the sail to its reefing points. Once the sail is secured, tighten up the halyard and slacken off the the topping lift and re tighten the vang.

[Clivert]

 further to my comment above, the gas strut in my kicker failed and without the topping lift the boom would be on the spray hood.
things you learn when you start sailing in small boat