If you suspect you're gonna turn into a hunk of bent metal sliding down the runway, I'd rather there only be a couple of gallons of fuel left, rather than a couple of hundred gallons.
In the scale of amounts of fuel aircraft carry, 30 minutes of fuel is "a couple of gallons". You don't want enough to overstress the airframe or fuel a massive conflagration if the tanks are compromised, but you also don't want so little that you've got no buffer in case something else goes wrong, and 30 minutes at cruise (or at the near-idle power settings used on approach, it isn't clear here) is a lot less at takeoff/go-around power.
