It looks like there's plenty of actual method implementations being filled out there, but they just happen to be in a .hpp file instead of a .cpp file.
A lot of the C++ standard library (STL) and Boost are header only (but not all)
This means to use the code, all you need to do is #include the header file wherever you need it, rather than including a header file and either building a separate static/shared library for the cpp file, or including it in your own build.
So that's really convenient.
Another thing that happens is that the by doing this, the compiler can potentially do a lot better job of optimizing the code by inlining parts of the library straight into your calling function.
With static libraries it is possible to get some optimizations like this made at link time, since link time code generation is a requirement for templates, but that's not possible with shared libraries since you are forced to call through an exported function entry point by their very nature.
Header only libraries are typical for containers or light weight libraries where the cost of inclusion and possible code duplication of code is worth the performance gains, and certainly very popular where templates are involved.
tl;dr, header only is cool just because they are so easy to consume and don't need a make file. Just #include and you are done.
e.g. if I had Hello.h and Hello.cpp, you'd need to either add Hello.cpp to your make/build or build a library and link to it.
Header-only version means all you do is
#include "Hello.h"
and you're all set to go.
Also, keep in mind that most of the library is templates, which need to go in the header files.
BTW, most of boost is header-only :) e.g. boost/noncopyable.hpp would give you a header only equivalent of the NoCopy class I wrote.
I've always felt weird about having to include some big-arse library for just using a simple container, so for things like this I prefer to write header-only versions.