What are rhythm charts for?

The rhythm charts are for drummers, as well as keyboards and guitars. A rhythm chart is like a lead sheet without the lyrics or vocal lines. It is slash notation, formally called 'Comping'. As Wikipedia describes: Comping (an abbreviation of accompanying) is a term used in jazz music to describe the chords, rhythms, and countermelodies that keyboard players (piano or organ) or guitar players use to support a jazz musician's improvised solo or melody lines.  By taking the vocal lines out of the staff, there is more room to indicate unique rhythmic notations that are beneficial for guitar, bass and drums.  Generally, these players are not singing with the band, so the vocal lines are not necessary.  The advantage of a rhythm chart over a chord chart is that there is more detailed information about the song map and the exact rhythms of the chord progressions.