I think theory is important as it becomes practical. I have sight read a lot of charts. I knew what chords to play and how to make them based on the theory behind spelling the chord. I was able to use substitutes at the appropriate times and in appropriate places because of an understanding of the chords.
For example, as a guitarist, when I see Em7b5, I also think Gm6, C9 ( no root).
When I see, Bb7+11, I also think Bb7b5, or F7+11 or F7b5. I have a range of possiblities for playing depending on where I am at the current moment on the guitar neck and what the bass player is going to play.