For those of us without those concerns, a mattress protector may not be necessary and could even reduce the comfort of the mattress.
You can mitigate some of that by making sure you are buying one that is described as “cool” in some way. That typically indicates that it allows air to flow, which means it won’t make you roast like some of the older types would. It would also be a good idea to get one that goes on like a fitted sheet, rather than the ones that go on like a pillowcase. The latter kind can turn the simple process of washing the mattress protector into a wrestling match.
So are they necessary? No. Are they a good idea? Well, yes, if they address an issue or concern that you have in your life. For me, the removable zipper cover on my Delarkin mattress is sufficient. I just take it off and wash it every now and then. If I had young children in my house, though, I might feel differently.