Rubber dam isolation is used routinely for most treatment, and always for root canal treatment. This allows complete isolation of the tooth (or teeth) in question from the oral environment therefore preventing contamination of the tooth and/or filling material. In this way it is easier to provide better quality treatment.
"Rubber dam offers substantial advantages and should be used whenever practicable. In addition to enhancing the quality of operative care, the use of rubber dam virtually abolishes saliva/blood splatter and aerosol. Non-latex rubber dam is available" - from current Dental Council Guidelines
Rubber dam isolation, while not widely used, is far from new - having been invented in 1864 by a New York dentist, Sanford C. Barnum.