[bad_user]

I upvoted your comment, as that's what I was going to say.

For example I just created a small script that synchronizes my photos on Flickr, but that also takes care of duplicates (that were tagged with a machine-tag representing an md5 of the file).

I could just keep it on my computer, and even lose it in case my hard-drive breaks, or I could improve it a little and release it on GitHub -- after which I'd have a backup of my script and maybe even some fixes for free.

Then somebody else might want to do this synchronization in reverse and add his own stuff to this little script.

That's how open-source happens, besides companies throwing projects over the fence, or doing it for the free publicity.