Roosts depend on the breed. It gives them a place to fly up to and perch during the night, and sometimes during the day. Some breeds will use them, some will not(silkies). Some like high roosts while others prefer a closer to the ground roost. I put a practice bar in the brooder at a couple weeks old that they use. It's only about 3 to 4 inches above the shavings.
I have Buff Orpingtons. They have had a stick sized perfectly for their feet since they were 3 weeks old. I've switched it out as they have gotten older. They never use it though. At night, we put them up and shut the door. They spend the night huddled around the door. We move them back and place them on the stick, but they re-huddle by the door.
My plan was to build a real roost this week. But, I'm beginning to wonder if it will be a waste of time. They are about 8-9 weeks now. One is 4 weeks.
I have two roosts inside of my coop made of old, thick wooden curtain rods and I painted large branches and screwed them in place in my run. The chickens sleep on the roosts inside the coop every night and spend a good portion of the day on the roosts in the run. They love them. It is also a good way for my hens to get away from a frisky rooster!