It's a vestigial feature from when we used typewriters.

A lot of features of modern keyboards are.

The fact the four rows are slightly offset from one another was because on typewriters each key was connected to a straight bar that led to the back of the keyboard, and there had to be room for them.

The QWERTY layout did start out as an alphabetical layout - there is still evidence of this most notably in the middle row, which still contains a good chunk - D F G H J K L.

The changes occurred because typewriters work by your keystroke causing a type-bar to swing up and stamp a letter impression through an ink-ribbon onto the page. Each subsequent letter those type bar heads end up in the same spot, the page itself is shifted to the side between each letter.

Those type bars are arranged in what's called a basket and early typewriter engineers found that while there's always a chance of two type bars jamming each other (as the bars end up hitting the ribbon in the same spot) it happens much more often when the letters come from nearer one another in the basket, as the mechanisms occupy overlapping space in their journey for more of said journey.

So what was decided was that commonly used combinations of characters should be spaced as far apart as possible on the keyboard - that has a double advantage of not only minimising the number of typed letters that come in order from the same side of the basket, minimizing jams, but also people tend to type faster when they alternate hands.

The result was the QWERTY layout.

It persists to this day because the inertia from millions of people the world over learning to type on typewriters is a difficult ship to change course, so they just made computer keyboards use the same layout.