Do they NEED to roost, no. But it's probably a little more sanitary for them to do so, so they aren't sitting in their own poop.
Have your chickens ever roosted (you didn't mention age or source)? Have you tried to train them to do so by picking them up after dusk and placing them on the roosts? Not all breeds roost as willingly as others (i.e. Silkies).
If you're wondering if there's an issue with your roosts or the configuration, post photos of them so we can troubleshoot.