What is a dental rubber dam used for?

Several reasons. improving the treatment field for the dentist and increasing the safety of the patient. Also isolating the area to be treated and make the treatment time shorter.
Isolate the tooth. It is a piece of ruuber with a small hole in it to allow the tooth to go thru. This isolates the tooth from the rest of the mouth. Most commmonly used during root canals to prevent bacteria from getting into the tooth and keep the disinfection chemical out of your mouth. It can be used to keep the tooth dry to allow adhesive to work better.
Isolate, protect. The rubber dam has been in use for over 40 years. It is used to create and maintain a dry field. This keeps saliva and, therefore, bacteria out of the treatment area. Also, it keeps the water spray and chemicals that are used to restore a tooth or perform a root canal out of your mouth. Helps us do better work.