I will often 'reinvent the wheel' on my own time just to gather some more detailed understanding of what's happening behind the scenes. I never really use the stuff in production code, though.

I used to code PHP and made a transition to Ruby at some point. After programming Ruby/Rails for a bit I decided to create a rails-like MVC framework for PHP. There are plenty of those in existence. I could have just read the ruby code, which I did a bit, but I wanted to understand a bit better why certain decisions were made.

Doing that little exercise helped out a good bit with that.