In the Watergate scandal , why is the press referred to as "the fourth power" after the legislative , executive, and judiciary powers?Is it a good...

Simply put, without the dogged work of the press to expose all the elements of the Watergate scandal, it would have remained secret. 

In a democracy, freedom of the press is an essential element.  In fact, I would argue that it is the most important.  The natural tendency of most organizations -- government included -- is to hide its mistakes and to keep sketchy things secret.  At times the Legislative Branch is successful in discovering problems, but the press is usually far more effective for  investigation simply because it is large and highly motivated to find noteworthy information.  Also, people on the inside tend to be more willing to provide information to the press because they aren't viewed as part of the power structure.