When is dental bone surgery necessary?

When a defect exists. Bone grafting in dental surgery is necessary when a defect is present. This may be minor or extensive, depending on the reason the graft is needed. If needed for implant placement, it may involve sinus bone grafting, block grafting, or use of a membrane to hold the material in place. If not for implants, it may still help to keep the topography of the bone and gum tissue.
Depends. There are numerous types of bone surgery. Can be to make a tooth longer so a crown fits better, can also be for gum work, to make it easier to keep certain teeth cleaner.
Depends. There are a number of conditions in which dental bone surgery would be necessary. Careful diagnosis of any bony or dental problems would determine whether or not bone surgery is necessary.