I've never had a need for one. If you ferment indoors, and the inside of your house is comfortable, the yeast will be comfy too, in a pretty wide temperature range.
But if you want one, heck, go for it. Different brewers like to geek out on different aspects of the process. If it makes it more fun for you, then go for it.
Oh don't give me none more of that Old Janx Spirit
No, don't you give me none more of that Old Janx Spirit
For my head will fly, my tongue will lie, my eyes will fry and I may die
Won't you pour me one more of that sinful Old Janx Spirit