Creative aperture = creative capture
More than just controlling light, your aperture isolates subjects with slice of focus
Shooting wide open, in this case with an aperture of f/1.8, creates a very narrow depth of field. With such a thin plane of focus, it draws the viewer’s eye towards a specific part of the image and renders everything else an indistinct blur.
By closing the aperture a few stops, this time down to f/4, the depth of field is still shallow enough to isolate our point of focus, but the background isn’t now totally blurred. Some people may find it more pleasing as a result.
When we stop the lens down to f/7.1, the background is now much more distinct. If having the context of the background and surroundings were important, like if we were taking environmental portraits, this would be a preferable choice.