My view on this is that it secures the engine nacelles. You'll have people running around out there pretty soon after and you don't want a fire risk, or a still running engine. Basically it's the quickest way to ensure a fully shut-down airplane.
Depending on the reason for the evac, you may have a busted airplane (gear not fully down) in which case you want the spar fuel valve shut to make sure it stays in the tanks.