Actually, the default screen is imprecise. With the advent of autofocus, modern viewfinder screens are designed to be bright even with slow lenses, at the cost of not really showing the difference in focus at fast apertures.

With the "imprecise" screen, it's hard to see the exact, "critical" focus needed for fast lenses. But autofocus doesn't care, so if you're using that, you still get accurate focus and have the advantage of a brighter screen for composition.

This isn't magic or anything; screens designed for manual focus show the difference better because they diffuse the light more, which also means that the brightness is reduced — and so the viewfinder seems dimmer. Since slow kit lenses are the norm, rather than yesteryear's 50mm f/1.4, and since the vast majority of users depend on autofocus, the default has changed.

But a camera with interchangeable screens lets you choose what you want even if your use is different from the modern majority.

Alternate screens may also feature focusing aids like split prisms or microprism regions. And, some have different patterns of grid lines for composition.