Need as in "fall over and die without it"? No.
Need as in "be a whole lot better of with than without"? Yes.
First, they just LIKE to roost on something, once they learn the alternative exists. Second, it gives mites a harder time getting a population going. And third and perhaps most important, it will keep the hens MUCH cleaner and thus healthier, because they are not lying around on poo half their lives. Which, even with good coop sanitation, they will be doing. Remember they poo a lot at night...
In fact if your coop is high enough it is a BIG quality-of-life improvement to put a droppings board under the roost that will intercept the poo (approximately half their daily output!) to keep it out of the bedding and make it really easy for you to remove that poo from the coop (scrape into bucket and remove, takes only seconds)
A roost is not going to take away any room from your coop. Either it is high enough for them to walk under it and use that floorspace more or less normally (like, it's at least 14" off the floor for normal-sized chickens) or it is just 4-6" above the bedding and they will hop up and over it and treat it as just sort of an odd contour in the floor and, again, use the floorspace more or less normally.
So, honestly I see zero reason for omitting a roost. You technically can keep chickens alive without one, but, really, why?
Good luck, have fun,
Pat