Depends on your definition of necessary. Chickens can survive a lot, so roosting is not necessary for them to live, but simply surviving is a very low bar to set. If you look at chickens from an evolutionary standpoint, they need to roost high off the ground to stay safe from land predators. We take care of that for them by (ideally) keeping them in coops and runs that are safe from predators, so they don't need to roost to stay safe. However, the chickens themselves don't know that. They still have the inner need to roost. It's an instinct, and it's independent of the actual level of safety of their environment. All chickens have an instinctual need to roost. Some breeds, like silkies, often don't roost not because they don't have the need/instinct/desire to do so, but because humans have screwed up their ability to do so. Silkies don't have real feathers, so their wings are crippled and can't do their job - can't help them jump/fly up. Silkies know their own limits and often won't even try, opting for the floor instead. Other types of chickens may not roost for other reasons - broody, sick, injured, too heavy, etc. But the default for normal chickens is to want to roost.
Roosting is better for them for other reasons, too, besides satisfying an instinctual need. Chickens poop a lot at night, so roosting keeps their butts off the poopy floor and keeps their feathers clean. Feathers matted with poop can lead to other problems, like fly strike.
If at all possible, it would be great if you could give your chickens a roost - any roost, even if it's a branch just a few inches off the floor of the coop. They'll appreciate it. As for roosts in the brooder, I always provide a low roost (branch) right away, and they start hopping up onto it by day 2 after hatch. They love playing on it during the day. I give them higher and higher baby roosts as they grow, until they move out to the coop, where I have roosts at different levels so that all birds can find a place to sleep.
Your chicks and chickens can live without any roosts at all. But they would be happier if they had roosts. So it's up to you to decide which needs you do or don't want to meet.