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The Rearm command actually dates back to the Windows Vista era. For all obvious reasons, Microsoft does not go out of their way to share the Rearm command, as it prolongs some people from purchasing Windows, up to an additional 90 days.
In all actuality, the default 30 days should be enough time for Windows 7 users to decide whether or not to purchase. Although, Microsoft does want the heavy majority of Windows 7 users to purchase after the initial 30 days, they do not frown upon or punish people that use the Rearm command, as clearly stated above; but, the command was not designed for everyone to use.
The Rearm command is what it is because Microsoft wanted to give the custom computer builders and IT professionals out there some breathing room. Often times, a system builder will install Windows on a PC for a customer. However, the customer may request some additional programs be installed as well. Most commonly, custom versions of Windows will include special software, drivers, help files, and other useful information.
Upon installing all of these custom programs, the system builder may have taken some time off of the customers default 30 day trial. Thus, it would not be fair to give the customer a custom built computer with less than 30 days to activate. So, system builders and IT professionals can use the Rearm command to gain back those extra trial days that may have been lost due to software installations and Windows customizations. Check out this article from Microsoft to learn more about the purpose of the Rearm command.