The stagger on a normal keyboard is a mechanical leftover. Typewriter keys were attached to levers that needed room to swing past each other, so the rows were offset. Nothing inside a modern keyboard needs that, but the shape stayed because everyone had already learned it.
Ortholinear layouts drop the offset entirely. A related variant, column stagger, keeps the columns straight but shifts each one up or down to match how far the fingers actually reach, which is why the middle-finger column usually sits highest.
The change is small in millimetres and large in practice. The keys most people reach for diagonally, especially C, V and X on the bottom row, are the ones that feel wrong longest after a switch.