Can you explain under what circumstances do dentists use a rubber dam?

Whenever is possible. A rubber dam used by dentists for moisture control, retraction and access, harm prevention, infection control and better visibility. The dentist should use the rubber dam whenever is possible.
Rubber Dam. It is considered mandatory for root canal therapy. Some dentists use it for all fillings and other restorations as well. It is used to isolate the tooth being treated, and prevent any debris, files, medicaments from going into your mouth and more importantly from being accidentally swallowed or aspirated during rct.