It's using relative line numbering, at least for me this is how I've had vim set up for the last 10 years. It makes commands like 10j easier to do because you can just look to the side and see how many lines away the target is.
I guess I can see the appeal; it's not for me though, I'm swimming in the local minimum of absolute line number dependence :)
I use relative numbers while in normal mode, and absolute numbers in insert mode. That way you get easy vertical jumps, but you still have a sense of your line numbers.