So when you add an external network, you pick a vSphere port group.
1. you give the information required to configure NIC cards to be attached to that network, so yes ... whatever is on the actual physical network.
2. some companies only want to allocate specific subsets ... maybe the initial set of IP addresses are free, because it's shared with the physical realm.
Later on, maybe you free up more ... but it's not one continuous block. then you can add a second range.
It's for flexibility, nothing more.