They don't, they're preview builds for insiders only, only RTM build goes to mainstream after all the preview builds
Creators Update (Redstone 2) won't go mainstream till March 2017 ver. 1703. Also All Insider Preview Builds expire and you need to install new builds as they are released. If your Build expires it will no longer boot.
I have one PC, and I am in the insider program. It is possible to have more than one bootable build on a PC. It is extremely convenient to have a bootable OS in a VHD.
I have been using preview builds as my daily driver for more than a year now. I have never had to revert or even been forced to clean install in all that time.
You do know this Build expires don't you ? Also as new Insider Beta builds are released you need to keep updating to the newer Build.
This right here it's crazy how these preview builds are stable as heck, most bugs that happen come from new features anyways...
anyone of you guys ever had on upgrading the problem: It could not be determined if your PC has enough disk space to continue the Windows installation. Restart Setup ever run into this issue i use gtp disk dual boot win 10 14393 and win 10 10586