They're there to criticize.

A movie about two guys kicking each other in the balls for an entire hour might become very popular with the public. But it's shallow, repetitive, and with no actual redeeming qualities.

The quality of something has only a small influence in its popularity. Critics are there to evaluate quality.

That's a pretty good point - sounds like the movie Jackass, which did pretty well across the board, critics and users alike.