One reason to get crowns is to restore the appearance of your teeth & face. Prior to preparing a cap, the dentist needs to restore a tooth that is severely cracked or decayed. Furthermore, since teeth support face muscles, anything less than a full tooth may affect your appearance.
Secondly, since a crown is the same size & shape as natural teeth, it will keep your jaw & bite aligned & prevent other teeth from shifting. Also, when a tooth is missing, other teeth tend to take on a greater share of the work of biting & chewing. A crown can help ease the burden for these other teeth & prevent unnecessary wear.