In don’t entirely follow that. It is 10 minutes from approval, and not 10 minutes from submission, if that’s what you mean.
So that the forum administrator can see the post as originally submitted. This might be relevant if major changes were agreed with the poster. There could be other scenarios. I don’t know whether the original, unedited post is stored anywhere, but I don’t think it can be found on the admin web interface.
The main problem is different, which is that changes within the 10-minute period are ignored when the email goes out. These might be changes that only occurred to the forum administrator after approval.