Why learn music theory? It depends many things: what do you want from music, what are the circumstances surrounding your musical evolution, how much are you willing to invest, what are your views on music, how you hear music... You can get along pretty well without it (Erroll Garner - still you will have to work your ass out to reach that level *Oscar Peterson was of the opinion that he would have been even better if he learned how to read music), but there are certains things we have to take into consideration:
1) Music is information divided in time and humans have a limited capability of storing big complex ideas inside their heads - the score has no such limits
2) Music is in a big part cultural and traditional phenomena, ideas build upon century after century of experimentation. You cannot build, from the ground up, centuries of knowledge by yourself, that would be a godly undertaking
So if you want ot travel the solar system you might be just fine without it, you could even go a bit further than the solar system, but if you want to travel galaxy (even if your main goal is the solar system) you will get rewarded with riches of knowledge that often go even beyond music itself. HOWEVER! Music theory is not composition as much as composition is not a bunch of chords and licks. Theory is the tools, the hammer and the anvil with which you build your creations, analysis of behtoveens sonatas will not make you a composer (though you certainly won't be worse off because of it), harmonic progressions and standart forms alone will also lead you nowhere.
What you do is write down tons of isolated exercises, play them and learn why such and such things work with each other and why others don't : some diminished chords might work in one situation, while in another it will feel very akward (it might be because of voicings or because of the stuff that surrounds it, let's not forget, chords do not hang in the air by themselfs it's what's around them that gives them flavor) and by knowing the theory, experimenting with it you know why it is like that and how to get what you really want and only after that you actually start to learn to compose. You have a bag of knowledge in you now you decide how you will build the whole thing - you have ingraved into yourself the relationships between tone, tamber and rhythm and this is when you make your own rules and brake the ones that aren't needed for your vision, because you have a foundation that is leading your ear, you don't have to think as much now, because you have internalized many things leaving your thoughts free to look at the big picture without distraction. But you stil have the hardest part ahead of you - finding your voice.
Of course everything has two sides and after some time studying certain branches of theory, it will make you think in a certain way, which might lead to you missing out on some discoveries that you would have found otherwise, so have that in mind, from time to time sit at a piano and just bang some random stuff, you might get unexpected results and with the knowledge backing you up, then you can analyse how that happened, or just leave it alone like it is, up to you.
So all in all, even if you are a producer of House music, Hip Hop or other branches that don't make as much use of traditional harmony and rhythm, you will still gain much needed insight into your field if you decide to brush up upon the good ole music theory.