I might be useful to emphasize the security aspect:
It Provides PC Safety
Routers typically have two built in firewalls. The first is simple NAT for network address translation, which isn't so much for protection as it is for simply making the Internet connection work to all your computers. The second is SPI or stateful packet inspection. Most modern routers have SPI, which keeps extra track of data in network packets and makes sure it's kosher (i.e., you requested it), protecting both the router and your computers by filtering out the bad.