IMO compilers are a bit like kernel development in that the real learning was the friends you made along the way, rather than actually being able to understand how a compiler works, which is a nice side effect. For example, when you write an OS you learn a lot about how code interfaces with hardware. A compiler is many people's first and only introduction to soundness and formal analysis. Being able to put on the hat of a compiler author can be useful in many fields that have nothing to with compiler development, and that's even if you go beyond "I just want to see what my code is being transformed into".