The xdp is a description of the form template (in XFA). This xdp file can be interpreted at run time by Form Server and rendered in real time (in fact in turns the XDP into a PDF). This is very useful if you want to manipulate the template before rendering (i. e. resolving fragments, adding templates together etc.). If you save the template as a PDF then the rendering has already been done and you have no ability to manipulate the template before rendering.
This is not to be confused with dynamic and static rendering that you have setup. A dynamic form rendering has the ability to react to the data that is being entered into it (adding subforms, manipulating the presence of objects etc). You can also have the form react to data being merged onto the template (this is true of wither static or dynamic forms). The static or dynamic nature merely describes how the form will react when the user is entering data.