I am having difficulty understanding why anyone would purchase the retail version of windows when the OEM version does not seem to be very different. For example, at the time of writing this, I see the difference on amazon to be $130 for the OEM license and $190 for the retail license.
After doing quite a bit of research, it looks as though OEM can only be installed one time, on one machine, and that this is the real advantage of the retail version, that it can be reinstalled multiple times.
Here's the catch. OEM copies CAN be reinstalled on different hardware. Microsoft even has support links on their website explaining how to do this. The key is to make sure that the license is fully deactivated and that only one copy of the OEM license is active at any given time. Link
If I am buying Windows 10 Professional for a small business and they need 8 copies for example, why would I buy an OS that is $60 more expensive per copy when there does not appear to be any obvious difference between them (other than maybe the OEM version may be more of a pain in the rear in the unlikely scenario that I need to swap out a failing motherboard).