Why are membranes used for dental implants? Are they necessary? Why do some dds use them and not others. Does it help with success rate?

Membranes. Membranes are not used for every implant placement. Membranes are used when you have a bony defect and its required for bone or ridge augmentation.
Better with membrane. A membrane its necessary to protect the bone graft, it prevents the soft gingival tissue from growing inside the socket and taking the place of the the bone.Some doctors and studies show better results with 2 membranes.Of course it cost more to do bone graft with a membrane.
Growth. Many are used to prevent epithelial growth down around the implant thereby preventing bone growth. Some will use simply as a barrier and containment surface.
Yes and yes. Membranes are indicated to promote bone healing k/a guided tissue regeneratio. Used when indicated to augment the deficient ridge, the grafted site with bone is covered with a membrane to prevent soft tissue invasion during healing. All dentists when indicated should use them. Yes, research shows improved success in bone healing.
Case. Just as no fingerprint is the same, no 2 patients are the same and some require procedures or steps that one patient may need where another may not. It's important to get a thorough examination and treatment plan to maximize successful.
Ridge Preservation. These membranes are very successful and are used quite a bit these days to promote sound bone formation. The membranes are usually removed at a later date, but sometimes they can be resorbed by the body and disappear all on their own. Ask your surgeon for more information.
Individual situation. The main purpose of a dental membrane's placement in dental implant surgery is to prevent the faster growing gum tissue from interfering with the formation of bone in an implant site where a bone graft is needed. No membrane is would typically be required in a long term edentulous site or in an immediate placement surgery where the implant diameter is bigger than the natural root's diameter.
Yes. Membranes , resorbable or non resorbables are use in aid to help bone graft material or normal bone bone to grow on an implant site without the possibility of tissue invading the area, the tissue grows at a faster pace than bone does and for that reason the need for the membrane in certain cases.
See below. A 'membrane' is placed over a dental implant site to prevent ingrowth of the gum tissue to the implant site. This tends to allow the bone to fully surround the implant as part of the healing process. It helps improve the long term success of the implant. Not everyone places membranes, however. Sometimes they are not needed if the implant fits in the bone very precisely.