If you have a missing tooth or teeth it is possible to leave a gap, however there are good reasons to replace a missing tooth:

• When a tooth is lost, the tooth that it used to bite against in the opposite jaw can grow outwards into a new position, and the teeth either side of the gap can tilt or drift into a new position. These new positions of teeth can cause new undesirable collisions between teeth when chewing and grinding that can lead to problems.
• When a tooth is missing, the other teeth close by then have to share the extra load, making it more likely for the remaining teeth to chip or fracture.
• Having more teeth allows for better chewing and food being better broken down. Chewing is the important first step of digestion and extracting the nutrients from food
• Of course there is also the cosmetic benefit of replacing a missing tooth – being able to smile and laugh without a dark gap being seen.