I love how the call-and-response approach allows you to reference more of an ensemble sound. More than just putting a few licks over a steady bass, you can start to sound as if you were two or more people, one of whom is laying down a groove while the other drops in all sorts of cool single-note commentary in between.
Imagine yourself as a harmonica-guitar duo, a band with a horn section, or even just the left and right hand sounds of a barrelhouse pianist, and you’ll begin hearing all sorts of possibilities for combining the steady thumb, your single-note licks and various kinds of double-stop- and chord-based riffs.
There are two more great things about playing in this kind of call-and-response vein. One is that it serves as a kind of automatic taste enforcer: if you’re spending half your time laying down the vamp figure, you’re never going to find yourself overplaying in the licks department. It’s hard to go on too long when you only have a measure or two at time in which to improvise.
The other benefit, particularly if you’re relatively new to improvising, is that it sets up some pretty finite spaces in which to do that improvising. I like to think of it as taking a wide-open field, where you could run anywhere, and setting up a relatively narrow set of cones to thread your way through.