Guitarists hate theory, but musicians love it. Understanding theory helps us make sense of what we hear and opens the door to musical freedom, composition and improvisation.

Not everyone needs to understand how to read notation, or learn about tri-tone substitution, but music (mostly) has form and structure - and is surrounded by harmony. Every guitar riff, melody or musical phrase has a place within the structure and harmony. Take away the structure and harmony and soon, all music would sound the same.

Structure is the time signature, the number of beats per bar, the meter that keeps one musician in time with another. Form is the number of bars , the duration of the melody before it repeats or changes to the next part in a song. Harmony is the sound of chords and other notes behind the melody that give the tune its context and feel.

One good example of how harmony changes the feel of a phrase or riff, is modes (Aeolian, Dorian, Phrygian etc.) Mode scales are just major scales starting on a different interval. So once you've learned a simple major scale, you have already learned all the mode scale patterns. The only thing you need to understand is how to use the modes - in context - what the harmony of each mode sounds like when chords are played behind it.

So if you play a riff from C major scale that contains the notes F, G and A only, and the chord(harmony) played behind is an E minor chord - then the combination of E minor and your three note riff would produce a Phrygian feel (sound) - a very different feel from the same riff with a C major chord behind it. Try it out!

Theory also teaches you that you don't need to learn hundreds of chords - a daunting prospect - and that you don't even need to learn 60 different scales. When I started learning to play jazz I was using so many different scales I almost forgot I was making music - it was impossible to relax and feel that musical freedom with so much theory in my brain. Nowadays, more often than not, I just use major scales or blues scales to create all the tension and jazz I need.

One example, without getting too technical - assume the only scales I know are the major scale and the pentatonic/blues scale. With these 2 scales I can make them sound like many other scales just by combining them with strong chord tones picked out from the harmony behind ( roots, 3rds, 5ths etc.).

So, if the rock band is playing an A7 chord which resolves to a D chord, I could play my D major scale, or I could play an A minor pentatonic or blues scale (even though the chord of A7 has a major 3rd). This works because the minor 3rd in the scale against the major 3rd in the chord creates strong tension, which is what makes the blues sound painful.

If I was playing over these chords in a Jazz context, I could just use a Bb major scale over the A7 - because it gives me even stronger tension. ( Jazz likes to do that)

A good way to get into music theory is to try and figure out what you're favourite guitarist is doing. For example, let's say you like Slash (GnR) and you want to know what he knows. Take the solo from Sweet Child of Mine - what scales does Slash use?, what are the chords behind the scales? Once you start asking these questions you get into the fundamentals of blues, modes and chord harmony. It's a way in to theory that doesn't involve reading music and heavy text books.