When you have all of your teeth they all work in unison with one another and each tooth supports the tooth next to it to prevent it moving. The opposing teeth keep each other in the right place and all of the teeth support the surrounding bone. If you lose a tooth then this delicate balance is upset.
Teeth either side of the gap can begin to drift in, teeth opposing the gap can begin to drift up or down and the bone that surrounded the tooth that has been lost can begin to resorb. In time, this can affect your ability to eat and chew and also have a cosmetic implication. Your delicate gum architecture can also be interrupted by drifting teeth.
It is therefore important to stabilise the bone and surrounding teeth using either a denture, dental bridge or dental implant to prevent these problems from worsening.