Excellent question! To be honest, the main reason is, I just feel the proper way is, regardless of what calendar day I'm doing this, to download an ISO once, why more? Don't worry, I didn't run out of time, yet. If I understand correctly, you are suggesting swapping hard drives between the machines. I didn't mean that, only something like simply sharing a vanilla Windows 10 ISO between separate machines via an USB stick. I also heard about the concept of genuineticket.xml before, but it didn't came up in four previous pages of this topic yet, and now maybe it's not the time to learn about it. OK, if there is no workflow of copying the Windows 10 ISO on a USB stick instead of having to download it to multiple machines, fine, I can work with the latter.
On win 8.x/10 systems with the os preinstalled it's in the bios called MSDM or oem : dm. Digital Entitlement is something else, it's stored at the MSFT servers based on the hardware id's from the system it's established on, the DE is valid for the lifetime of that hardware, and you can establish more then 1 HWID for it, all will be saved by MSFT.