One reason people like to use non-internet-routable addresses on their internal networks is because they are non-internet routable. So the risk of some misadventure exposing your internal network to the internet is, at least in theory, reduced. And if you are using NAT, then it's comforting to know that your private addresses on the inside really are as private as they can be. There is no one 'magic bullet' that can prevent unwanted exposure/access to your network (ask Sahirh), but there are a lot of little steps you can take. Using a non-routable address range is one of them
