Off the top of my head, scientific publishing. Keep it locked down until you have published then open source it.

I personally wouldn't want my work code online, as its always rushed and there is a fair bit of technical debt I would like to to refactor, rather than having potential employers thinking that I always write shit code. Feature requests are valued more than code quality.

I suppose wanting to hide perceived 'shoddy' workmanship is a reason.

I say, let it all hang out. Code can be continually improved.