Why I think everyone should learn at least basic theory, I know a lot of you will say well Guitar player X doesn't know any theory and look at how well he's doing" but I'm here to tell you that famous musicians who don't know how to read a note of music are the exception rather than the rule. Most musicians, if they want to communicate with other musicians-to play in a band or to teach them their songs-have to know at least the basics about how music works. These basics-notes, chords, and so on- are what we call music theory.
Notes and chords are the building blocks of the language of music. Music theory defines the many different ways you can arrange those blocks into songs and compositions. Without theory, all you have is noise; applying music theory, you can create great works of art.
Musicians apply music theory every time they sit down to play or sing-whether they know it or not. When you read a piece of music, you're using music theory. When you write down a series of notes, you're using music theory. When you play a chord, you're using music theory. When you sing a harmony line, you're using music theory.
Even those musicians that don't have any formal training use music theory. When they put their hands on the piano, they might now know that they're playing a major ninth chord with the fourth in the bass; they do know that those notes together well, even if they can't tell you the strict chord construction.
Now, if they did have formal training, they could go beyond that just playing the notes to sharing those notes with others. Instead of pointing at their fingers and saying "play this," they could actually write their notes and chords down on paper, in a format universally understood by musicians the world over. After all, it's a lot easier to tell someone to play a CM9/F chord than it is to say "put your first finger here, and your second finger here," and so on.
The knowledge of how different notes work together also helps you expand on the simple melodies you're currently playing. When you know theory, you know how to accompany a melody with chords and how to voice those chords so that they sound good to your ears. You also can learn how to turn that simple melody into a full-blown arrangement for groups of voices and instruments, and how to create your own melodies and compositions.
Without knowledge of basic music theory, you won't be able to fully express your musical ideas; nor will you be able to share those ideas with others. Of course, it isn't just professional musicians who need to know music theory. Even if you're just doing it for your own personal enjoyment, knowledge of theory will help you better appreciate the music you play or sing.