When trying to learn a song, you can anticipate where that song will go simply because you understand the rules that guide us all. Don't look at music theory as a boring exercise that can limit you. Don't think that by learning and knowing music theory, it will turn you into a generic player without a unique sound. Think of it as the key that can set you free. A key that opens the door to a vast tool chest of ideas that you build from to develop your own unique sound.
Music theory helps in the communication of a musical idea.
Communication like that is all subjective and rarely accurate in describing the part. Intervals Intervals are the relative distance between two notes. There are basically four types of intervals. They all have to deal with how many semitones up or down the neck you move. They are called Major, Minor, Augmented, and Diminished.