When this setting is checked, if the main windows is put in the upper left corner and closed, the next time it will open with some offset to the bottom right.
The implementation of the GUI in macOS has a few shortcomings. The problem is, we can’t fix them ourselves because it depends of GTK.
I hope the new GTK 4 release will fix all these issues and won’t take too much time to be integrated in Siril, fingers crossed 
Perhaps, but there are no plans to integrate Siril into GTK4 for the time being, as there is so much work to be done.