More importantly, it's a learning tool. It's been part of many courses and many rounds of Google Summer of Code. It's an embeddable kernel + OS free of the cancerous GPL, under a permissive MIT licence. It is extremely well documented, well tested, and consistent. Writing drivers for it is very simple. This makes it interesting for education, for-profit development, embedded applications, etc...
It's also just fun. There is a very active forum[1], wiki[2], and community[3] that has been spiting out hobby OSs for a decade. The kernel originally used in Haiku is from NewOS, a project by Travis Geiselbrecht that was also just a hobby project.
[3]: irc://chat.freenode.net:6667/#osdev