One of the major differences between like, your PC, and a high-availability server is that high-availability servers require those things as well—and get them, piecewise, while still running. It's like doing a complete overhaul while the plane is flying.

IBM mainframes, famously, phone home if they detect a faulty component. Service personnel will be on site, same day, to replace it before you even knew it was there, let alone had the opportunity to ask.