3560 QoS

I was wondering if someone could help me better understand queue mappings. The below is from the Cisco 3560 QoS guide.

 Configuring the CoS-to-DSCP Map
You use the CoS-to-DSCP map to map CoS values in incoming packets to a DSCP value that QoS uses internally to represent the priority of the traffic.

 Configuring the DSCP-to-CoS Map
You use the DSCP-to-CoS map to generate a CoS value, which is used to select one of the four egress queues.

If the above is true, why do we need four maps for queues?

  cos-input-q    cos-input queue map keyword
  cos-output-q   cos-output queue map keyword
  dscp-input-q   dscp-input queue  map keyword
  dscp-output-q  dscp-output queue map keyword

Wouldn't we only need 2 or 3 since the newly formed CoS forms the map to egress queue? When would we ever use the dscp-output-q? I assume it all comes down to what you are trusting but this statement confuses me:
"You use the DSCP-to-CoS map to generate a CoS value, which is used to select one of the four egress queues. "


Thanks for your input