A washer-dryer manual says the inlet hose should be screwed on so that
it points down at about 45° from vertical (either way) but not
vertical or horizontal. I'm just curious --- why does that matter?

(As is usual, the hose supplied has a 90° bend just before the
screw-on coupling, so the hose has to be parallel to the back of the
machine but could "point" in any direction in that plane.)