I would say no, you don't technically need one. Having 11 birds, I'd say about half of mine sleep on the ground and the other half sleep on the roosts. (Unless of course I put them all on the roosts when I check them at night).
That being said chickens naturally will want to roost in a higher location. Goes back to in the wild type of thing where it's safer in the air than on the ground. A good example of this is wild turkeys.
Are you thinkinh or planning on getting more chickens? I'm sure your one now would enjoy a friend or two.