What is the benefit of opening port 8333 to a client?
If you don't want to, you don't have to open ports at all. You'll still have a full node, it will just have fewer connections. Full nodes usually make 8 outgoing connections and can have many more incoming if you have the port open (for a total of 125 by default). Incoming connections can also be SPV (phone) wallets, or other full nodes.

So the benefit is that you're helping out the network a little bit more providing connection opportunities and bandwidth to more other full and non-full nodes. There's not really a shortage at the moment though.

Typically only the first time when you run the software it might do a DNS lookup on the domains you mention. Each name represents dozens of IP addresses of random other full nodes (not just nodes run by those people). Once your node has a connection to a few peers, it will learn new IP addresses from those, maintaining its own list of IP addresses which is stored when you close the software shuts down.