[boost] Why Boost.Build?

Hey everyone, Is there a reason why Boost uses Boost.Jam/Boost.Build for reasons other than tradition? Does it have advantages over other build systems, in particular CMake (which I consider to be its main competitor)? Are there many people/projects who use Boost.Jam/Boost.Build outside of Boost? These questions are asked purely out of curiosity due to my ignorance of Boost.Jam/Boost.Build, not because I have an axe to grind. Cheers, Greg