If you had orthodontic treatment then you will definitely need a retainer. This is primarily because your braces straighten your teeth by pulling them into their correct location, an action which is achieved by loosening the ligaments and connective tissue between the roots of your teeth, the gums, and the jaws. When your braces are removed, these fibrous connections are still loose, meaning teeth are at high-risk of moving back to where they came from and undoing all your good work. A retainer will ensure your teeth stay put, reducing any risk of movement. Essentially, the will help you retain your new smile.
Talk to your specialist orthodontist about how to retain your new smile after you have completed orthodontic treatment.