If a component has a prerequisite configured, then you are essentially letting the system know that this component should not begin until the pre-requisite has finished. So if a user attempts to activate such a component, the system will trigger a prompt window to confirm that you wish to activate even though one or more pre-requisite tasks are not completed.
Once you have addded some pre-requisites between your components, the Schedule tab on your job will reflect this by staggering your components:
You don't need to order the components in your workflow according to their pre-requisites - but your workflow will be easier to understand if components are ordered this way!
As a component is checked to be a pre-requisite, it is added to the Pre-requisites tab.