One possibility:
The firewall shutoff in these aircraft seems like an odd afterthought IMHO, and have caused fatal accidents in the past. The fuel selector is capable of turning the fuel off, but Cessna installed them in a manner in which the "OFF" detent cannot be used, so that's where the firewall valve comes into play.
The lever of the firewall valve attaches to a wire core type cable and the bolt used has come loose in the past and the firewall valve worked its way closed inflight on its own. Also, the sheath of that cable has also slipped in its mount on the firewall which has also pulled that valve from the fully open position.
My only guess as to why they set the fuel system up this way is to prevent accidental fuel starvation when switching fuel tanks. The firewall valve knob is supposed to be safety wired to the “ON” position using a breakaway safety wire.
