Quick, picture a stereotypical Twilight fan. Now picture a stereotypical unemployed 20-something. Now a megachurch pastor. A ruthless CEO. An overworked stay-at-home mom.
Chances are, you and I had similar pictures in our heads. A weepy-eyed fangirl writing fan fiction in her Team Jacob shirt. An unshaven slacker sitting in his mom's basement playing Starcraft at two in the morning. Joel Osteen. Gordon Gecko. This poor woman.
These pictures, the ones that sort of exist in our collective cultural consciousness, are tools that good writers and speakers use as a point of connection with the reader or listener. In other words, sometimes, stereotypes are good.