Finally, note that the transistors protect the fuse, not the other way around.
The crowbar fires because the transistors have already failed, or are about to, and the only thing the SCR does is blow the fuse faster while keeping output voltage lower (hopefully).
Why would you need that?
The crowbar is ONLY relevant, if you are powering a bunch of circuitry that absolutely cannot be subjected to excessive voltage, lest it cost tens of thousands of dollars in servicing to fix.
You saw crowbars often in classic and mainframe computing. It's rarely used today. With everything consumable and unserviceable* today, such a failure results in needing new equipment anyway, so why bother?
*That is, a single failure is basically BER (beyond economic repair). You can if you want, but you'll pay almost as much for a replacement board as a new unit. Or you can do component-level replacement, but now you need orders of magnitude more knowledge and capability, which isn't cheap either. (If you can amortize that over many fairly-quick repairs, you get something like what Louis Rossmann does, but keep in mind also that most of his repairs are stop-gap, no-guarantee, "it runs again but you want to backup your files and move them onto something newer" sort of thing. As-new repairs are possible, but that doesn't mean you want to do them. Corroded traces and burned up PCB can be repaired, but it is truly a heroic process!
Tim