If the user has a high-resolution widescreen monitor or a multi monitor setting, it does not make sense if the user has to scroll up and down to revisit the preview while not using the available screen space.
In this case, the user need the preview on the left (or right) side rather than above or below the edit field.
Further, a popup-preview would provide additional freedom in window arrangement.
Note: This can also combined with live preview, so the non-experienced user can see in real time which effect his edits have. Specially if the currently edited paragraph appears in the (almost) same visual height as the preview of the paragraph.