It adds libraries that allow for a more modern approach to C++ programming.
In my experience many C++ programmers are really the early 1990s C++ programmers, pretty much writing C++ classes, not a lot of use of generics. The more modern approach uses generics to compose software together in manner thats more like dynamic languages, yet you still get type checking / performance in the end. It is a little bit ugly to look at. But once you get over the syntax issues it really is quite nice. Boost gives you a lot of the tools you need to compose stuff easily. smart pointers, functions, lambdas, bindings, etc. Then there are boost libraries which exploit this newer way of writing C++ to provide things like networking, regex, etc etc...
if you are writing lots of for loops, or hand rolling function objects, or doing memory management, then you definitely should check boost out.