it creates a sense of depth and focus. of course it should not be abused, but if you don't use the white gap, the object in the background will look like it's connected to the object in the foreground. if you're using an establishment shot and the foreground object is tiny, you need a big white gap, because otherwise you lose focus. it really depends on the ratio between foreground and background material.
in panel 3, for example, if the gap isn't there, the guy kneeling will look like he has his face on gintoki's arm, kagura will look like she grew out of gintoki's arm, and the window will look like wooden bars sticking out of gintoki's shoulder.
in the last panel, the white gap is not really necessary, but i put it in anyway because i felt like it.
in the second to last panel, a white gap should've been on there because the background is farther away than gintoki in the foreground, but i didn't put the white gap in there because gintoki's outline is thick enough compared to the lines on the background, and that's enough to give him focus.
that's another thing, you can also regain focus by making your foreground items have thicker outline than your background items. that's why i said the white gap is completely arbitrary and it all depends on what effect you're trying to achieve.