Not everything needs to be built from source, true, but certain software that isn't in wide distribution may have source as the only option.

Or maybe some tool hasn't been properly updated and errors on your computer, maybe you can debug it and change a small amount of source code to fix it. Maybe the source release is far ahead of the stable binary release and you want to test or use new features.

If you download the source for something like linux or ffmpeg or your favorite emulator, you will learn a whole lot by doing a deep dive.

However. Gentoo. Have you ever built firefox from source? That shit contributes to global warming. It takes so much time and CPU power to build such a heavyweight application from source and the tangible productivity benefit that you get from compiling on your own machine rather than downloading a binary is far outweighed by waste from the sheer active CPU and real time spent building. Maybe if you had a threadripper distcc setup, and only a dial-up connection to update source, it would be faster to compile everything than to get new binaries all the time. But for everyone else, if all you want to do is use the software, downloading binaries for the most popular applications is the way to go.