Ive been playing about 4 and a half years now and I dont consider myself to be that great. Ok, I can play some Trivium, Metallica etc but nothing itoo tricky.

Now what I really want to progress into is shredding. I can play some little shred licks from Trivium songs but I wanna get better.

Now practicing scales, chromatics, legato etc is a given, but why oh why do you need theory??? Surely if you can play the instrument, you dont need to understand the mechanics of music?

Im not saying theory isnt needed, I'm purely wondering what benefit it is (apart from in song writing).

Help me out anyone?
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You don't NEED theory, but it helps alot. I have a friend who's a great guitar player and he knows very little theory.
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Surely if you can play the instrument, you dont need to understand the mechanics of music?

that is precisely why one should learn theory.

knowing theory helps you transfer that knowledge from one instrument to the other, and why and how you are doing it.
It's important to learn the 'guidelines' of theory but equally important to throw them out the window.
Well consider this, you want to shred, so when your flying around the fret board... how do you know what notes to go to next??? well you can just play the scale paterns up and down at razor speeds ( like some of those clowns ) or you can follow the chord progessions, know when to change scales, throw in arpeggios, etc. Plus, if you know some theory, then it helps your ear out alot, because you understand how all the music fits togather. Im not saying you need it, but i would reccomend it.
Listen to a solo that is written over a basic scale, like stairway to heaven solo, compared to a solo that is just notes all over the place that young people do when they first get a guitar. You don't need a whole bunch of theory to shred or play guitar. Take Yngwie for example, he only uses 2 main scales, phrygian and harmonic minor.
Its the difference between being like M.A. Batio or Steve Vai. Steve Vai is very musical, its not all about shred, its about sounding good and interesting, it sets a mood while M.A. Batio is fun to watch for about 5 minutes because he never does anything that sets a mood besides ok this guy is fast as hell and it all sounds the same. morale of the retarded comparison, It doesnt matter how good u know the instrument until you can make people enjoy watching you play it...or something like that.
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Get all you theory behind you so you DONT have to think about it when playing, it comes as second nature and you can easily add anything new you learn to that pile.

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Take Yngwie for example, he only uses 2 main scales, phrygian and harmonic minor.

Though I bet he knows all the rest of them too
If all you want to do is learn tabs, you don't need any theory at all.

If you want to learn how to improvise, theory will give you the structure to do it
with skill (as opposed to trying random notes to see what sounds good).

It doesn't have to be a huge task. For rock/blues improv you can get by with
a fairly minimal grasp of theory. Just the very basics will get you up and running.
As time goes on, you may get deeped into it, and you'll get more skill as well.
I am a beginner and I think that if you want to learn how to play a guitar you should try to learn everything about it the more you know the more potencial you have to get better I havent done any music theory YET but intend on doing so when I get a little more comfortable with my guitar

but thats just how I see it
If you ever want to play with other people or write a song then you need some theory. Too many people ask this question and the answers are always the same. You don't necessarily need theory but if you want the hours of jamming to get a good basis for a song to turn into about 15 mins of planning and thinking of a chord prohgression that works then you need theory.

Theory + Little Actual Playing Skill = Good Music
it makes writing music easier. and someday i'll guarantee you want to.
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The benefit, my friend, is that it is experience of hundreds of thousands of musicians who knew more than you, written down, so you can easily absorb it instead of spending thousands of years experimenting to come to the same results and understanding of how music works.
He who joyfully marches to music in rank and file has already earned my contempt.
He has been given a large brain by mistake, since for him the spinal cord would fully suffice.

Remember: A prudent question is one half of wisdom.

Yeah, I need to start learning some theory. I can see how it helps a lot.
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Every little bit of theory you learn adds something to your playing. Every guitarist wants to progress right? Learning theory is a great way to take your playing to another level. Not to mention it will bring your level of enjoyment way up when you play a lick that fits perfectly with the chord progression and you know exactly why. While not every great guitar play may be a walking theory dictionary every great guitar player has developed his "own" theory per se. For example a blues/rock player doesn't need to know every note of ever mode of every major scale when learning how to manipulate a pattern will work just fine.