Rubber dam allows for extremely effective moisture control and preventing contamination of the operative field. This is particularly important when working with hydrophobic materials, such as composite. 

It also helps prevent contamination from other substances such as blood and debris. 

Rubber dam isolation lets the clinician focus more on the tooth/teeth they are working on. This also makes it easier to carry out treatment without having to worry about surrounding anatomy. 

Rubber dam prevents bacteria going from a carious/pathological lesion to the rest of the mouth. It also will help prevent bacteria getting in to the tooth and reducing the prognosis of treatment e.g. if the pulp is exposed.

Rubber dam will help prevent substances and equipment going in to the patient’s airway. The classic example is that of endodontic files that may accidentally be dropped. 

Similar to airway protection, the other tissues in the mouth sometimes require protection from substances that we use e.g. sodium hypochlorite.