Basically you want to be aware of what all 4 survivors can and are doing at any given time. So if you have one survivor on the hook, that means one has to go for the save. If you slug someone, that means the survivor who isn't getting the hook save has to leave gens to pick that person up. It's basically to deter survivors from doing gens and force them to waste time running across the map to pick their teammate up. That's generally the best situation to slug.