This is a pretty common surgery in people who have no lower teeth. If you stick your finger in your mouth and put it in the space between your lower cheek and the ridge and teeth, it is is the mandibular vestibule. As people age and have worn dentures for protracted periods of time, the edentulous ridge itself actually gets smaller, this is over years pressure die-back from wearing dentures. As the ridge gets smaller the stability to lateral movement of the lower denture is compromised. By deepening the vestibule through this procedure, (very common) they can gain some additional stability. Of course this can't be done over and over as the mandible eventually becomes very thin (commonly referred to as a pencil thin mandible) and is prone to fractures.

Vestibuloplasty is sometimes also used to create a flap to cover the ridge in implant procedures. A thin layer of the mucous membrane is reflected backwards after a three sided rectangular cut in the vestibule, and then folded back over the top of the ridge. (split thickness skin graft). While this leaves an exposed area of uncovered subcutaneous tissue behind, it is usually OK, as it heals in by secondary intent, ie. on its own without any special procedure to cover it up.