I originally trained as a classical piano player but learned all about chords playing clarinet in a jazz band. When you're improvising away you need to know just which chords are being played. The guitarist always had a book with the chord sequence for every tune so if you got stuck you could always take a peek. Only in practice though. Wouldn't do to get stuck in a performance otherwise they threw stuff, beer bottles etc. So you develop the instinct to pick out the right chords even if you don't have the music. Now I am a heavy user of fake books - just the melody and the chords. Main reason is you get so many tunes - 1000 or more per book so you're likely to get most of the tunes you want. So a big thanks to Hal Leonard.