In college, every performance major was required to study a secondary instrument. Guitarists studying jazz were allowed to count classical guitar as their secondary instrument. I've always enjoyed playing classical guitar, and was even a classical guitar major for a semester at Indiana University. My classical chops aren't what they used to be, but I've been revisiting several of the pieces I used to perform as part of my current practice regimine.
Classical guitar has helped me in several areas:
1) Right hand technique. In my everyday playing, I use a hybrid style picking--pick between thumb and index finger, and use my other three fingers to pick strings as necessary. Classical guitar essentially super-charged what I could do with those otherwise idle fingers through various attack techniques and the simply be strengthening those muscles.
2) Left hand technique. Simply put, classical guitar is all about effeciency--getting around the fretboard with as little effort as possible. This increased my endurance for long practice sessions and helps me stay relaxed on intense gigs that require a lot of difficult playing.
3) Posture. In high school jazz band, my teacher once suggested the sax section all slouch in their chairs so they'd play with a better feel. He was joking of course, but there's definitely a casualness about how jazz musicians might lounge about when they play. I've spent many hours hunched over an electric guitar, and it takes a toll on my back and neck. The best remedy is to pick up the classical guitar and run some scales. Somehow the feel of that guitar reminds me of the constant corrections from teachers and I snap back into form.
If you're a guitarist that's never studied classical repertoire, I encourage you to give it a shot. Find a good instructor, tell him or her about your current level of experience, and see if they can work with you on the weak areas of your playing. Not only will it make you better, but there's a wealth of great music to discover for the classical guitar.