From what I've read, it's the barking that scares potential intruders. More than anything, they don't want neighbors or the occupants to be alerted.
I saw a show where a reformed burglar was showing what to do and not to do for home security, and he said he hated small "yappy" dogs the most. He stayed away from homes with barkers.
But I have to think that a quiet, big dog who snuck up on you and gently grabbed your ankle in his jaws might get your attention, too!
Didn't Mythbusters do a show where they ended up concluding that some tasty meat pretty much distracted even trained guard dogs?