It's great when you need a bunch of roughly parallel lines but don't want to spend all day making one line at a time. The only drawback is that it can be hard to control the beginning and end of the line. It's the kind of tool that's really helpful when you want to make a big mess. I've used these brushes with ink when I want to have a sort of impressionistic or out of focus looking background.
It's also a great way to build up interesting layers of grass or foliage with watercolor. It has come in handy many times while working on Harrow County. In the image below, I laid down a flat area of green and then went over it in darker colors with my spikey comber, building up layers of grass.
These brushes cost between $5.50 and $12.50 depending on the size. It's not a tool that I use every day but it's a tool that I always enjoy when I do use it.