Whole layers of management have been eliminated because there is no longer a need for people to screen data, produce information, and send it up to higher-ups. IT took their place and does the job better. Managers at different levels may use any type of information system. For example, middle managers may sometimes access TPSs to check certain facts. However, in general managers use ISs as the text describes.
Information is power.
Therefore, employees often try to have access to as much information as possible, and try to deny access to others. When there is no rationale for doing so, they are engaging in politics. You need to be aware of the politics of developing and using ISs.