An example of situational awareness being useful in non-terribly-time-constrained situations (to address a point made by several commenters) is that if you notice what people around you wear, you'll be less likely to unknowingly signal stuff you don't want to signal through your clothes.

EDIT: Another one: if you're going to some place with somebody, and you don't pay attention where you're going, if you later want to go there on your own, you won't remember how to get there.