Why do dogs bark?

Spot loves to bark at the kids passing by his gate, this happens frequently since the gate is close to the street:(

Sometime I go outside to see if anyone if out there, most of the time I see the kids but once in a while there is no one.

http://www.dogproblemsolutions.com/why_do_dogs_bark.php

"Why do dogs bark at nothing? They don't. It may seem like there is no reason for the barking behavior, but that is not always the case. When you feel that your dog is barking for no reason, start to consider the possibilities. Are they hurt, hungry, ready to go for a walk or have them spotted someone outside your home? We may not notice some of these smaller things going on around us, but your dog will. If your dog is barking at the walls or at something non-existent, there my even by a psychological or health reason for the barking, so you may even want to have your dog checked out by the vet. "