In a standard office LAN environment why would I NOT want to tick both Register this connection and Use this connection?
How do they differ and in what situation would I select only one without the other?
SnapOverflow
When you connect a NIC to a network, windows registers it with the DNS server using the IP/IPs of the NIC so other computers can easily find it by name. If you have a PC with two NICs (multi-homed), you may want only one of them to register to the DNS server so other computers trying to connect to your PC do it over the IP on the main NIC. Unticking the first checkbox on the second interface does the trick. This is of course not an standard configuration that can be used for many different purposes, but then again, that checkbox is not there to be used for standard configurations... This is actually quite useful when you have multi-homed servers.
The second one forces Windows to make the registration using the provided DNS suffix vs for example the one received from the DHCP servers, this is specially useful when you have multiple domains but you want your computer to register to the DNS service using a specific one instead of the one received from the DHCP server.
In short, on a standard office LAN you want the default values (only the first one checked) and everything should work fine as long as the network is properly configured (DHCP server passing the proper local DNS Suffix, etc.).