Games are expensive, gamers need to know if a game sucks or not, and one of the ways to know that is by game reviews. Comparing games with movies is not really the best comparison because if the movie sucks, you only lose $7 (based on movie tickets on my country), with a game you lose $60 (you can return it if you want, but not everyone has that option).
The problem imo is not the reviews themselves, the problem is that people take too much attention to the score and not the actual content inside it. The score of one game could be 7 out of 10, but if the points that are taken do not bother you (by looking at the actual content of the review) then there's no reason for not to pick it up just because it's a 7 rather than a 9.
Gameplay videos and demos do help making your choice, but unless you watch a whole walkthrough, there will be stuff that you may not like about the game that were not on the demo or the gameplay videos you saw.
Either being a poster opinion or a reviewer opinion, you need to see what they like and didn't like about the game, and by their opinions you can make you own choice, not by simple looking at a number.
Nintendo and PC gamer