In all my years of playing guitar, I've occasionally filled in and played bass. I enjoyed it at the time, but never had any plans to buy one. I started dabbling in recording orginal music at home and decided I needed a bass. Since I bought my own, I've found myself listening to the bass lines of songs instead of the guitar. This is a whole different perspective of music for me.
For me (and I would imagine most of the people on this board) starting to play bass was one of the best things I ever did.