Every tooth in our mouth is very important. Every tooth in the mouth is flanked by a tooth on either side, every tooth occludes with a tooth in the opposite arch, and hence the biting action takes place. The teeth, having lost the support of that missing tooth, tend to lose their alignment over time, and this can lead to crowding, spacing or other alignment problems. Wisdom teeth are the only ones which doesn't occlude with a tooth in the opposite arch.
Front teeth are used for incising / biting the food, and they give you your beautiful smile. They also support the muscles of the face - we have all seen the sagging of the face when a particular front tooth has been lost. When a front tooth is lost, the lips tend to sink-in at that position. A missing front tooth can mar your features and cause an imperfection to your charming smile. Back teeth are used for chewing and for giving fullness to your cheeks. They also maintain the proper posture of the jaws - loss of teeth leads to wrinkling of the face and over closure of the lips.
Bone support of the adjoining and opposing teeth, thus lost, is difficult to replace naturally by the body, but can be done artificially by the dentist. The space lost due to tilting of adjoining teeth and over-eruption of opposing tooth, results in decrease in space for replacement of a new artificial tooth. Failure to replace a missing tooth can result in all the problems explained above. Replacing a missing tooth at its early stage will avoid all these problems.