Short answer: no, not necessary at all.
Depends on your TV and trailer. I used one when I was towing our 5,000 lb TT with a Jeep Libby diesel, which had a 1430 lb payload. It made a huge difference in how the combination handled.
I don't use on towing that same TT with a BMW X5 diesel with a 1260 lb payload limit, and don't miss it a bit. But the BMW has self-leveling (air bag) suspension.
So, long answer: it is not necessary sometimes, and sometimes it is necessary. Which, is a function of the trailer and the tow vehicle, as well as how said TV is equipped as well as what the tongue weight of the trailer is.