After playing a bunch of Ludum Dare games, I gave a lot of feedback on what the developers did good and what they could improve upon. I had a lot of different things to say, but my most common criticism was the lack of music. Something about it just made the game feel wrong to me. I understand that the developers were constrained on time, but music is crucially important in games for a variety of reasons.
To set the tone
Music has a variety of tempos, pitches, and rhythms that give it a happy cheerful vibe or a sad and depressing one. In games, it is hard to portray a tone in visuals alone. You can see an industrial wasteland or a cotton candy world, but you don’t really know how to feel. The music is meant to cement that.
When you play games like Zelda or Skyrim, you feel like an adventurer because of the grand and upbeat music as you venture the world. Though when you come across a dungeon or cave, the music gets a bit darker as to signal the scary unknown. This makes you “feel” the game, being afraid, happy, or sad. When you start feeling these emotions, the game has you hooked and keeps you playing. It is all meant to make the game complete and create a true experience.
To reinforce the gameplay
Music not only helps with the emotional aspect of the game but also with the gameplay itself. Have you noticed in Mario all the different types of music? Notice how most of the music is fast. This is because the gameplay is timed, and you are meant to go fast. You know those star power ups that run for only a short time? The faster music helps emphasize that the ability is special, as well as signal its end. Music serves as a tutorial and an extra signal when used in tandem with the gameplay.
Without music, games would not be the experience they should be. Music helps invoke emotion into the player, as well as guide them through the game itself. No wonder why AAA studios spend thousands on famous composers and huge sound teams, because it is such an integral part in making a great game.