Business requirement for a SharePoint App

Ok, I know what a SharePoint App is but what I cannot wrap my head around is under what situation should I develop an App and under what situation should I develop a normal SharePoint Web Part.  So far based on the research I did and the logic I found I would select a SharePoint App under the following conditions

1. The component is isolated and does not need to interact my with the parent site and its lists.
2. The data stored by the component is just localized to the component itself and the other sites do not need to access the data stored by the component.

I would develop a Web Part if

1. The component needs to interact a lot with the parent SharePoint site and other sites.
2. The data captured and stored by the component is needed by other sites.

Is this correct?