Hi Erick. By adjustable topping lift, I am referring to a line that is fixed at the top of the mast, which runs down to the outboard end of the boom, through a fixed block and then forward to a cleat. It is not the main halyard. Basically it holds the outboard end of the boom up when the sail is dropped.
When we bought our S boat, Firefly, it did not have a boom topping lift. Whenever we dropped the sail we had to have the boom crutch in position and try to drop the boom into the small target of the crutch. Otherwise we had a heavy boom crashing on the cabin top or into the water, or worse on someone's head (which fortunately never happened). Yes, it would adjust the height of the boom for a boom tent.
The only disadvantage of having a boom topping lift is when you are sailing you need to keep in mind there is a line running from the mast head to the end of the boom that could catch on something like a piling or a buoy or another boat if you sail too close. In a decent breeze such an event could be bad news. I always have a knife on hand to quickly cut the topping lift if that ever happened, but I am very careful to avoid close encounters--especially with an 84 year old boat!