They also provide more information than a simple abstract number glowing on a screen. Watching that needle instantly snap back after taking a deep breath from the system just before rolling in is quite useful feedback.
WAI can do all that, too, plus allow your computer to give you more info that a mechanical SPG can give you. WAI (meaning, the computer that is receiving it) can show a dial with a needle that reacts just like a mechanical needle....
Actually, according to NetDoc, his computer display makes it WAY more obvious than a mechanical SPG needle does if you are breathing off a tank where the valve is not open all the way.