a hub assigns ever computer on the network a different IP..
but if you're trying to share an internet connection from you're cable company, they only give you 1 IP adress. A router can take this IP adress and internet connection, and then create a sort of private network and share that internet connection. In essence, letting every computer on the network share the 1 IP adress, and give them a private network IP at the same time.
a router hub also is a lot slower than a hub. A 10/100 router allows each computer to go at 100 speeds. a 10/100 router has to split up the speed between each computer, so 4 computers on a4 port hub 10/100 hub may only actually be getting speeds of 20-30mbps each.