It allows two people to have better posture when they're both hacking into the mainframe on the same keyboard.

It allows you to keep your arms shoulder-width apart which makes it easier to avoid hunching.

Even just a keyboard with left and right halves angled toward the respective arms would help. Because with a typical keyboard, the arms come at it at an angle, and then the wrists are crooked to have the fingers on the straight key rows. An angled or split keyboard allows to keep the wrists straight.

Same as the ergonomic mouse. Natural poses are less stressful.

Which means palms facing each other instead of facing down in one case, and hands should-wide apart with relaxed shoulders instead hunching in the other.

Caveat: Most people nowadays never learned to properly type but are self-taught. Which makes using split keyboards more challenging.