Yes. I learned so much after I started reading source code. I discovered the history and implementation of many language features, including the more obscure ones which are the subject of many fun stackoverflow questions. I obtained a deeper understanding of the threading model. I developed a good sense for the performance cost of my code as well as the optimizations I can expect from the implementation. I was able to create libraries in C that can be loaded at runtime by the language. I was able to embed the language in C projects.

This isn't something beginners should do, of course. It is a fact that it does lead to a much deeper understanding of things.