One reason (among many) to do this is because compilers require complex and demanding source code across a wide-range of theory and algorithms. And so make goo

One reason (among many) to do this is because compilers require complex and demanding source code across a wide-range of theory and algorithms. And so make good tests for both the source language (is it sufficiently expressive to do this cleanly?) and for the various analysis, optimization, and code-generation passes.