This is extremely useful, not only for inspiration and learning, but also so you actually know what is happening under the hood. This is great for troubleshooting and for reasoning about your own code. This is one big pro for open source in development!

I learned a lot by reading the source code for Python dicts (which also comes with a lengthy motivation for why it was implemented that way) and any Haskell library (Hackage links from the manual page directly to the source code for the respective function, which makes it very easy to see what is happening).