Ok just read this
If a filter uses the Image component but no Selection component, Filter Forge considers the filter to be an image-based effect such as a distortion, and combines its output with the original image by linear interpolation using the selection as an interpolation coefficient, so that the filter output is restricted by the current selection
My assumption that it would have no effect on a filter seems wrong, but does open up new possibilities

but one point is you can't get rid of the selection without closing and reopening FF