Makes getting the drop on them easier.

I can never figure out why people never learn this lesson. I can see it now. Forty local people have been killed by a vampire. The sun is setting and one mope says to the other: "Let's check out the basement of this castle!"

Because sunlight can kill a vampire, really quick.

I'm not sure how it is in eastern European folklore, but in the novel "Dracula" by Bram Stoker, vampires were able to move about during both the daylight hours and at night, although I think in the daylight their "powers" were reduced. It was the brilliant silent film "Nosferatu" with Max Schreck that introduced the concept of sunlight having the ability to destroy a vampire (by the way, if you've never seen this classic, you absolutely should, and what better time to watch it than on Halloween?).