When bone loss occurs around a tooth there are times when the gum tissue is lost with it. This is how gum recession occurs. Gum tissue is very dense and is attached to bone. For these two reasons it is a very good protective barrier from bacterial infection in the mouth. It provides a tight seal around each tooth preventing penetration. If bone loss is severe enough sometimes the seal becomes broken allowing bacteria to penetrate the remaining bone quicker. It is for this reason a gingival graft may be necessary to restore that seal.
There are times it may be for cosmetic reasons. If it is noticeable when the roots of the teeth begin to show root coverage procedures may improve esthetics. There are always two goals during soft tissue surgery:
1) Function. To put the protective gum tissue back so that further destruction will not happen.
2) Esthetics. Although results vary depending on the severity of the defect achieving root coverage will always improve how teeth look.