One reason for this requirement is that you want one SIM for your work number and one for your private number, so that you don't have to carry two phones. (You could forward one to the other as a workaround, but that doesn't help with SMS or outgoing calls.)
That's exactly the use case for me. I own a business and don't want to give out my personal number for anything related to the business side. Helps me stay organized. I know when a call is coming through I know the nature of the call before I pick up.
