Genuinely: why? All that is related to the business email is already accessible, it's just been forwarded elsewhere. The info is already known. What's to discover?
It indicates that you used your this other email for work purposes. The point is that they don't know what's in there, and they want to see if you discussed stuff relevant to the case. The judge will deny such a request as a fishing expedition if there is no basis for believing that you used your personal email for work purposes. But if it is discovered that you started sending work emails to that address...
I'm not suggesting this is actually a problem, but that's how an argument could go.