There's an old IT proverb:
It's not DNS There's no way it's DNS It was DNS
It's a useful to remember how much DNS does for us. It is the backbone of nearly every single system on a network. Sure, from time to time you might be doing stuff over straight IP addresses, but chances are you are highly reliant on DNS to make sure things are communicating properly. Active Directory makes this twice as important, because a large portions of it require properly configured DNS to function.