Nope, you don’t need a seal/sleeve to make it operate... I use it without my sleeve often.. but I’d be lying if I said it didn’t help things fit more snugly.
In the past, I’ve had pussy or mouth sleeves and that fit even more snugly. The less girthy you are at the base of your dick... the more you’ll need a proper sleeve or seal.
The same can be said for shaving. It will fit a lot more smoothly if you’re clean-shaven or trimmed. Hair can get trapped between the tube and your skin and that lets air escape the vacuum tube. If you apply more lube, it can make a tighter seal and compensate for the hair.
The idea is that the tiny spaces that are created by hair, allow air to escape, and are instead replaced by lubricant, thus giving a tight seal.
I’ve been letting my bush grow because I’m tired of shaving and I must admit that I haven’t gotten sealed in nearly as tight as I did when I was smooth-shaven.
Hope that helps!