In 'Fallout 4,' there is a gross line between the healthy vault dwellers and the decrepit, rotting beings that have been hanging out above ground. In the continuing S.P.E.C.I.A.L. video series, players can see that Agility is the key to keeping that line enforced and is key to shooting, dodging, and being stealthy.
When the lights go out, you'll want to be the first to strike, and the last one to walk out.