Just think of it as showing the aircraft is potenially dangerous to ground personnel.
You want both conditions, doors and engines, made safe before people approach.
Slides being blown onto you is perhaps less lethal than being sucked into an engine, but I wouldn't want either to happen.
Had a catering truck try to attach during pushback once. Beacon on, tug giving lots of black smoke as it took up the load, aircraft starting to move and the number 2 starting to turn, none of these would stop the man on his mission. Scary or what?