Unity builds are a very old and widely established concept. I dare to speculate they are older than the regular header and source file split, as C required a full blown preprocessor to split code in separate files and also someone had to come up with the include guard trick to get around the limitations of including the same definition multiple times.

They aren't employed that often because there are some tradeoffs and in large projects it's benefitial to split them in smaller translation units to limit the scope of incremental builds.