The answer is that it is very important. Without music, movies would devolve back into silent films; not that there's anything wrong with them. Though, I think we can all agree that both the soundtrack and score of a movie are integral to making it what it is. Music has a number of effects ranging from far and wide: it has the power to make us feel sad and depressed, happy and triumphant, and even like we're in the moment with the characters; but the most important thing of all is that this emotional roller coaster ride creates indescribably amazing feelings which proves that sometimes, the best way to describe something is by not saying anything at all. It's like when a friend asks you about your opinion on a movie you just watched and the best response you can come up with is "Just watch it. Trust me you'll thank me later." However, I am going to attempt to contradict myself right now by putting these feelings into words; wish me luck.
Let me start off by introducing you to a composer that you've probably heard of and personally is my favourite in the business. Of course it is the maestro of music himself... Hans Zimmer. If for some reason you are drawing a blank on his name, let me jog your memory. He's worked on great movies such as: 'Sherlock Holmes' (movie series), The 'Batman' Trilogy, 'Man of Steel' and the most recent 'Amazing Spiderman 2'. His track record is very impressive to say the least, but the reason for this is because he never ceases to amaze. He has this way with an orchestra where he is in control of your emotions every step of the visual journey depicted on the silver screen; but enough of words, just listen for a moment and tell me what you think.
What did you think of 'Time' by Hans Zimmer?
Now you've heard me talk about how music affects the tone and mood of a movie, but let me discuss what a lack of music can do and how it highlights its necessary role. Some of you people must be thinking, "What? That's crazy, since when does a film nowadays not have some sort of music in it?" I'll tell you when, it's when the director doesn't make the best of what they're given. What I'm talking about is, for example the content of an action movie centres around explosions, gunfire and end of the world stuff. Typical, right? Then in the screening room, the director is told by the producer that its missing something, and that's when it hits them.
"I'll add an awesome guitar solo on loop in the fight scene".
"Alright, cool. Which one?"
"All of them!"
That's a sin in my book. One song can't sustain the entire movie. Why would anyone think of doing this? Linkin Park doesn't solve everyone's problems! Depending on which of their songs is used it sometimes does the opposite, but that's besides the point. My point is that directors shouldn't ignore something so important. In fact, sometimes the repetitiveness of guitar chords are great in action films because they're simply awesome and used to the best of their ability. It's about not forgetting music and ending up using it as a slapdash finish during post-production. It needs to be layered throughout the movie to take full effect.
Let me give you an example of another composer who worked on some films. His name is Steve Jablonsky, and he orchestrated the soundtracks for the 'Transformers' trilogy as well as the Age of Extinction movie coming up (can't wait!). They are all directed by Michael Bay meaning you immediately know its an action movie with explosions, stunts and a hot actress (Megan Fox), so lets compare it to my earlier example.
Musical-wise, my personal opinion of the first and second movies were that they were a stroke of genius. It is the perfect example of not only having a fantastic musical score, but then having the director applying it in the right scenes to change the tone and mood. Thus, he uses the score in harmony with the cinematography to perfectly capture each moment.
However, I feel that the same was not accomplished in the third movie. It just didn't feel right to me. After the movie I had to think long and hard about remembering if the musical score was evident in 'Dark of the Moon'. I tried not to confuse it with my recall of the soundtrack in the other two movies and excluded songs during the credits. After all this, I remembered one song from Linkin Park in one scene... and that was it. "Where'd you go Steve?", I thought to myself. The answer to this is that he was there, it's just that Michael Bay shoved him behind a curtain, probably because he was focused on other things (like replacing Megan Fox *sobs* and consequently changing the script to work around this new character played by Rosie Huntington-Whiteley). That's just my opinion.
All in all, it comes to show that music defines movies in a big way in that they share an inseparable bond which rely on one another to achieve greatness. If anything, sometimes it can be underestimated and/or unappreciated as sometimes it is treated like ambient white noise in the background when it shouldn't be.