If you are getting a crown to keep a cracked tooth together, the crack or fracture could become worse if you don't get the crown. Fractures in the teeth can lead to tooth decay or even infection in the innermost part of the tooth, because bacteria can leak into the crack and infect the tooth.
If you are getting a crown for cosmetic reasons (for a stained tooth or badly shaped tooth that makes you feel self-conscious or embarrassed), choosing to not get a crown just means that your tooth will have the same appearance.
If you are nervous about getting a dental crown placed, talk with your dentist. Having a conversation with him or her could go a long way toward making you feel more comfortable with this procedure.