TL;DR as explained by bstinson
Basically the problem is, starting on Day 1 we have the os/ repository populated with GA packages *and* updates (0-day updates and those that came in while we were in the middle of the build process). os/ will continue to receive updates.
If you use the os/ repository, your install will use those updates as part of the install, but some folks would prefer to install using GA content, and perform updates later. That's where the kickstart/ repo comes in.
The kickstart/ repository is a snapshot of GA content, and will not change until upstream does another point-release.