Normally because you have had a tooth extracted or had one knocked out, leaving you with a gap in your teeth. Obviously closing the space is important for cosmetic reasons, but your dentist also has good clinical reasons to fill in the space, too. Leaving the gap open often puts extra strain on the teeth either side. It can also affect your bite. The teeth on either side of the gap may come together into the space and alter the way that your teeth meet, which can cause further problems.