Why are keyboard keys staggered?

In every keyboard that I have used, the keys are arranged so that the keys are staggered a small increment to the left from the previous row. Q is above and slightly to the left of A, and A is above and slightly to the left of Z. This pattern continues across the entire board. Case in point:

What is the motivation behind this design? I would think that a column layout would be easier to type on - and indeed, many ergonomic keyboards seem to go with a column layout, where Q is directly above A and so on.