This is one of the most common questions I get.
Some of the implant “supercenters” advertise that it is not necessary. Unfortunately that marketing is misleading. It is a case-by-case situation that needs to be carefully evaluated.
If the bone is not wide or tall enough for an implant, a bone graft is recommended. Sure, an implant can just be “put” in into almost no bone. Is that really helping you? What most fail to understand is that you are compromising on the essential long-term foundation of the implant, which is the surrounding bone.
What if you were constructing a building that was to be secured by four main posts in the ground? If you took out one main post, making the building stand on three legs, it would still stand. If an earthquake happens, would you rather be in the building with four posts or three? Dental implants are no different; they are subject to extreme forces on a daily basis.