Music influences the way you feel, and that in turn influences the way you think and thus the way you act.
For example, when you're feeling a bit down, you can put on your favorite music and presto! You feel better and are more inclined to take on something.
This is the way music is used in films, but perhaps an even better example would be corporate video and commercials, where the makers want you to act in a certain way.
Using the right music greatly enhances their chances to achieve their objectives. This by the way shows why using "stock music" on corporate videos is generally a bad idea since it's generic and doesn't target the audience's emotions like specifically composed music can.
Also, it often forces the editor to adapt his cuts to this generic music, which detracts from the efficiency even more.