Personal pleasure of an alternative desktop. Experimental playground for OS developers. Minimalist box for older hardware with high performance. Obscurity benefit against hackers that I hear PPC Mac users still get.
A few things like that. It's not competitive against Linux/BSD desktops in about any other case.
That's what I like. For when even a good Linux distro is too much for the hardware. The slow, old thing can be transformed into a nice, new thing.