In general I think that a good musical instinct is much more valuable than any good theoretical education. So, if you’ve got that and you feel well the music and the tension in it, you would need less the theoretical knowledge than people without this gift. On the other hand, knowing harmony for example makes it much easier to read scores and learn pieces, simply because your eye is faster to catch the harmony and your brain doesn’t need to work hard on recognizing every note separately, knowing how to read the pattern instead. If you are interested in knowing how to analyze a piece and understanding its structure, I would highly recommend you to study Basso Continuo, in English it’s called Thorough-Bass. It will help you understand the connection between playing music and the basic theory it’s build on.

Thorough-Bass is the skeleton and harmonic structure of all pieces. It used to be a very popular way of writing music during the baroque era - to have a bass and numbers under it would tell you which harmony you are in, then you could choose the way you would like to perform the part (in chords, arpeggios, using 3 or more notes as it suits the music and style best). On top of the Thorough-Bass you got the high solo voices (like violin or flute). Also music in the Classical and Romantic periods is built on this idea of skeleton of harmony, but after the baroque period the composers have become more precise about the way this skeleton should be performed and left it much less to the choice and improvisational freedom of the performer. It explains why improvisation was so important and the base of good musical education up to the 19th century.

Now the tradition of improvising has disappeared and became a matter of rare musician’s specialty (I’m talking in classical playing, except for people specializing in authentic performance who study basso continuo, otherwise, improvisation survived the 20th century only through Jazz). I’m not a big book reader on those subjects but I had a friend who swore by the treaty of Johann Mattheson and C.P.E. Bach and I noticed this book: The Art of Accompaniment from a Thorough-Bass By F. T. Arnold, which might be interesting.

 

 

Hi Ram,Edna's right, I think, that for solo playing, instinct, meaning your ear and understanding of style, is probably the most important thing. However, theory, when well-taught, not as a bunch of arcane or abstract rules, but almost like an x-ray to music's brilliance, can offer a way to get inside the score w/ a depth of understanding that is pretty powerful. For instance, your instincts might tell you what the most climactic moment in a piece's surface is, but there can also be structural events worth being aware of, even if they are understated. That might be part of the meaning that you'd want to share. If you're aware of the multiple formal designs in a piece like, say, Chopin's 4th Ballade, you can choose to bring out the different structural arrival points in different ways. Being aware of how Beethoven makes the recap in Appassionata so unique through the unresolved dominant pedal is part of its power. You develop a heightened awareness of motivic relationships (something particularly handy in Brahms). Your sense of line is heightened w/ a study of counterpoint (obviously useful for Bach and counterpoint in general). Study of harmony sensitizes you to chord function and resolutions, the shape of a line, the drive of a progression, and the fabulous ways in which expectations can be deflected. Recognizing ways in which composers play with expectations, both locally (unusual harmonic progressions) and structurally (manipulations of, say, sonata-form norms) can give you insights to interpreting works w/ depth and authority, not just a clean and attractive surface.(Remember that Renaissance painters risked grave punishment dissecting corpses to get beneath the surface.) Knowing how to study a score gives you authority, reasons for your choices beyond just- well, I like it that way. It can be a powerful teaching tool. At least, I hope some of this is true, or my years studying and now teaching theory to musicians (many of whom are pianists) are sadly misguided... (Btw, I see you're from Israel. I got my masters at the Rubin Academy in Tel Aviv- some of the happiest years of my life. :)  )

What helped me by learning music theory is I gained the ability to not write the same song repeatedly. I learned why I preferred certain sorts of styles and other ways to express myself with endless variations. The other advantage to music theory is you can communicate to other musicians what you are doing. All the music that you are writing "accidentally on purpose" will become a skill.
There's a guy in San Francisco, CA. who wrote a book simplifying music theory education  with his biz partner. I've had music lessons from him (and Lynne Vanne) previously, and they both show in this book their many insights and simplifications. For instance, Mr. Natural taught me that the different kinds of chords (major, minor, ninth chords, dominant seventh chords, etc.) have certain emotions and feelings connected to them, (something which I knew by instinct but had never been aware of.) He showed me how different styles of music use different chord structures and how harmony works in those various styles. He taught me about "altered chords" that I was using in my compositions, but I didn't know what they were or how I could use them skillfully. He taught me to improvise and know where I was going with what I was inventing as I was doing it. He taught me to be comfortable playing in any key.
All you have to do is buy this book and do the lessons he's got in the book and you'll have completed a college-level class in music theory in less than half the time.