@klaatu listing individual features misses the point; the idea is that if the software you use every day can't do "view source" on any command you run, it's defective. if you can't edit the source and see the effects of your change immediately: also really missing the point of software.
#emacs only happens to be the most widespread instance of this philosophy being applied, and the one with the richest history and active community.
@technomancy This is a really solid answer to the question. @klaatu
@technomancy @klaatu expanding on that point a little bit, there's also tremendous value that comes from the dev culture building a tool where there is a view-source button on everything: that is, every mature codebase is designed to encourage and support that.
You get useful, informative docstrings, customizable variables *with explanation*, things like the org-mode info pages (which I think get way too little love for the value they offer). All part of it.