Between one of our wooden windows and the wall there is a gap of about 5 cm that is only very loosely filled with plaster (see below). The craftsman who did some other plastering work in our house suggested to cover the gap with a thin wooden panel instead of fully filling the gap with plaster. At that time I thought it would be a good idea and did not ask for the reason, but now I am wondering why, especially since I expect the plaster would provide better insulation.
Is there a reason to not fill the gap with plaster?
Edit: In Germany, I am used to windows that are recessed. In such a configuration, there is no gap as shown in the following photo.