Braces and orthodontic treatment are used to correct “bad bites,” or malocclusion (teeth that are crowded or crooked). In some cases your teeth may be straight, but your upper and lower jaws may not meet properly. These jaw or tooth alignment problems may be inherited or could result from injury, early or late tooth loss, or thumbsucking.
Read more here http://www.mouthhealthy.org/en/az-topics/b/braces