Dental implants are a form of root replacement therapy. It is necessary if you lose a tooth due to an accident, cavities or if you had a tooth taken out because it was non-restorable.
An implant can be described as multi-purpose support for various restorations. One possible restoration is a crown/cap that replaces a single tooth. A second example is a bridge, where two or more implants can replace three or more teeth.
Another example is stabilizing a denture, so it does not move unless you purposely remove the denture. It will definitely save denture wearers from embarrassment due to it falling out during the most inopportune times.