Why is an OS with only a ready and running state inefficient?

Question

I'm studying for an operating systems exam and I cant think of a sufficient reason as to why the system would need more than two states. Could someone explain the reason further to me?

This is the question as its asked:

Answer 1

You would also want a 'waiting' state. When a process is waiting for some I/O input or input from another process, it would move to the waiting state and another process could run while it is waiting.

This might help you some: http://people.csail.mit.edu/rinard/osnotes/h6.html

Answer 2

What if a process wants or needs to sleep/delay/wait for some time/event?

Waiting for I/O to finish is a common example, which can be as simple as waiting for the next key press by the user or waiting for some client to connect over the network.