One such way was the psychological effect of the horde - the feeling of helplessness when alone and confronted by zillions of bloodthirsty creatures trying to rip your guts out. It's clear after a bit of playing that there's just too many to handle by yourself - even if you can shoot a few of them and run from some others, a single hit will stop you dead, meaning you'll quickly get overwhelmed. There's also the subtle reminder from horror movies that the Good Guys always team together to survive, and the Bad Guys are the ones that run off and try to make it alone (remember Burke from Aliens? Yeh you do.).