They're defacto standards. Not formal standards. The fact that you can compile and run a GTK program that will run on most linux and even other unix like systems without problem.
It means that someone creating a program, proprietary or open source, can, with little effort run that program on most linux distros.
We have quite a handful of internal GUI tools, some use Qt, some use GTK, written in python and some in C or C++. The GUI stack makes up the biggest stack here, but they use a few other libs as well, e.g. cURL, openssl, gsl.
All these are readily available, and we run these on various versions of RHEL, Debian , Ubuntu and FreeBSD beacuse of the defacto "standards" provided by these environments. Even porting these (at least some of them) to windows, where many of these libraries are available would take too much time because of minute details - there's other real problems we need to focus on. We hope we still can run these programs on Ubuntu in the future though, but it will not take that much to make it infeasible