Missing teeth can be very detrimental to one's appearance and one's oral health. A missing tooth in a dental arch is similar in effect to a missing brick in an architectural arch. Arches are extremely stable, but taking out just one brick or tooth can destabilize the entire arch. In a building, this would lead to a collapse of the archway. In a mouth, this could lead to drifting teeth that could be lost due to tilting (like the leaning tower of Pisa) or periodontal disease associated with difficulty cleaning poorly positioned teeth.
There are multiple ways to replace teeth. The ideal solution is dental implants because it gives the new crown (or tooth) support. Another solution is a bridge, which requires crowning the adjacent teeth and have them share in the stress.
Lastly, when multiple teeth are missing and finances are an issue, a removable partial denture is an option. This device is removable at night and feels less natural than an implant or bridge.