I suppose the obvious answer is because we want to see if we’re wasting our money. But who are these critics and do they have the same taste in movies as you do? I remember watching Siskel and Ebert and thinking how awesome it would be to get paid to watch movies and tell what you thought. Even though the idea of Siskel and Ebert amazed me, I had no further connection to the show whatsoever. From what I notice about real movie critics and the average Joe is that they are from totally different planets.
The average person watching a movie is looking to be entertained. They want to know who is in the movie, if it was scary/sweet/funny/etc, and if they should go and see it or not.
Your average movie critic probably doesn’t watch movies the way you and I watch movies. They are looking for the director’s execution of foreshadowing, and camera work along with the consistency of the mood and how the score mismatched the big climax. I for instance couldn’t care less. I want to know if it was entertaining, was the movie like any other movies, was it worth seeing at the theater or should I wait for HBO. There have been too many movies that I thought were terrible that the critics praised (including Brokeback Mountain which was nominated for an Oscar) and too many movies that I thought were great that got poor reviews. The Critics are more designed for film students and movie insiders for the most part than they are for you and me.
Some user based review sites like Flixster and Rotten Tomatoes are more of a promising endeavor, but even they can have reviews from jaded movie patrons. Your best source for reviews at the end of the day is you. If you like the trailer, go see it. Who cares what people say!