I think Boost was very successful in encouraging development of high-quality libraries that seamlessly work together (most of the time) and have consistent build infrastructure. It is not that important nowadays because the most important parts have already been integrated into the standard library and we have a de facto standard build system (CMake) that makes it easy working with small libraries that do one job well.

Where Boost is a bit lacking is modularity and somewhat varying library quality but this is kind of expected considering the number of subprojects.