Its sad but apparently true, apparently Dwayne Johnson just being advertised as being in a film can help sell that movie. Johnson has so far appeared in some decent movies but also some real bad movies. But guess what all of them have been sucessful at the box office why is that other then the fact that Dwayne Johnson has a fan following.
In terms of revenue, movies that feature popular actors gross more at theaters than independent movies or movies with new and less known actors. Because Dwayne Johnson has been so successful in his acting career, any movie that he stars in will gain revenue at its release. The movie, however, may not be as successful over time.
Dwayne Johnson has made a very good career post-wrestling. He is certainly a hit-maker when it comes to movies. He clearly selects movies that are potential blockbusters. The new "San Andreas" movie will probably be a huge smash hit. Eventually, the career trajectory of muscle bound, action movie stars starts to plummet, though.
For tent-pole films it is crucial that proper casting take place, but even with a star such as Dwayne Johnson, that presence in a film such as San Andreas does not mean the film is going to be successful or a financial windfall for studios. The film needs to be solid and entertaining as well as have star power.