The purpose is to create an item on a kind of "MAC address blacklist". If you create a static lease for that MAC address (or client-id) with block-access=yes, DHCP requests from this client will be rejected. If you do that after the client has already received a normal lease, the client's IP address will remain valid until the next renewal attempt from the client (which should come after 1/2 of the lease time expires), and the client will get a reject on the renewal attempt.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.