Are dental implants only for missing teeth?

No. Dental implants can also be used as anchors to help stabilize dentures when all the teeth are missing and, as the other doctor noted, as a temporary means of anchorage in orthodontics to help move teeth.
Mostly, yes. However, there are times with an implant is recommended when an existing tooth is deemed to be hopeless. At that time the dental implant may be done at the same appointment.
i would think so. If you are not missing any teeth why would you need a dental implant ? That said, perhaps you are referring to their use in supporting and retaining a removable denture. Still, teeth are missing. Orofacial implants can be used in the reconstruction of the face : i.e. An ear prosthesis is an example. These are not dental implants although they are the same implant, but not used dentally.
No. There r other uses, but primarily they r for replacing lost teeth.
No. Dental implants can also be used for anchorage in orthodontic treatment. While teeth can move within a person's jaw, dental implants cannot; therefore they serve as great anchors when moving teeth.
No. While missing teeth are a great reason to get dental implants, they can also be used for other things including orthodontics. Ask your dentist if you are considering this for yourself or google it if it's for trivia ;-).