A cleaner finish than you get by hand
Windows cleaned by hand almost always show something once the light hits them. A streak in the corner, a bit of haze across the middle, a spot that dried too quickly. A window vacuum lifts the water off the glass before any of that can set in, so the pane comes out evenly clear the first time. It is the kind of finish you notice on shop windows or in freshly cleaned offices, where the glass almost disappears. Most people do not realize their own windows can look the same until they see it.