The most common reason for a dental bone graft is to prevent bone loss following a tooth extraction. These are called “socket grafts” because the bone-graft material is inserted into the empty socket left by the extracted tooth.
Specifically, it is your alveolar ridge (along with periodontal tissue) that will shrink down in the months and years following tooth loss or tooth extraction. Your body actually allows resorption of the bone material when it no longer receives the day to day stimulation exerted by an active tooth or dental implant.
A large portion of bone resorption takes place within 3 to 6 months of tooth removal. The vertical aspect of your bony alveolar ridge (attached to your jawbone) tends to disappear faster, but the horizontal aspect (width) can also shrink.
Sockets that get bone grafts, and relatively quickly after the tooth extraction, end up losing over a millimeter less ridge bone height and over 2.5 mm less bone width. That may seem small, but the fact is that even one or two mm extra bone height/width can make a big difference if you ever want to get a dental implant. It could make bone graft surgery unnecessary before getting implants and make the operation less painful.