If you use
connect, You'll only connect to those specific nodes.
addnode On the other hand, try to connect to the specified nodes, but keep connecting to others too (unless you add more nodes than outgoing connections, I suppose).
It may be useful with respect to privacy to connect only to the nodes you trust; How to find them is another question.
Also, as far as I know, there are no coded pairs in the official bitcoin binaries.
You can find the sample file bitcoin.conf here for more information.