With a simulator, there are always missing commands and programming errors, and it can never really be as complete as the real IOS. However, because an emulator runs the IOS, you get every feature ...
Imagine a lab pod with 4-5 routers, connected with serial links, Frame Relay, and Ethernet. It has PCs for testing with ping and traceroute. Now think of the cost: $0.00. And it takes no more ...