When you use CTIOS Cisco provides you with a full set of development tools: ActiveX, Java CIL, .NET, etc. With this tools you can develop your own Agent and Supervisor desktops or integrate the CTI features in other applications, CRMs for example.
With CAD you have got an Agent and Supervisor desktops as it. You can add some CTI functionality to the agent, but with this approach you won?t have the ability to integrate CAD in third applications.
You can use CAD in enviroments when an easy to deploy appolication for agents is a must and you don't want to make complex CTI integrations.
If you need to add CTI features to third party applications or develpo custom interfaces for agents, supervisors or realtime statistics you would use CTIOS.
Hope this helps,