The eBay blocking rule is there to protect you as much as possible, not to hinder sales. You would never block a good customer, but perhaps you have someone bid and win, then not pay. Well you can block them to make sure they can no longer bid on your listings.
You can also block entire countries from your listings, that is of course if you ship internationally. For example, African countries such as Nigeria are well known for the love of electronic item scams, so you can block them and stop any drama up front.