Learning: I learned a big deal of web development (HTML, CSS and JS) from view-source.
Tweaking: I sometimes fix broken closed-source/proprietary applications or websites by checking the source and writing a user script.
Those two can probably fall into the "Nerd" category, though...
When I was a kid and started doing that, I thought that the compiled scripts were written like this, and I was fascinated (and terrified) by how complex they were.
I recall being both impressed by the complexity and either annoyed or impressed by how well formatted the raw HTML output was.
"How can they possibly stand to work on this when it's all jumbled together on one line??"
I would manually pretty-print so I could actually read what was going on, and now I still like to output tidier HTML when I can, even if no one is ever going to see it.
