Why do firefighters have dogs with them on the job?

The dog most often used on a firetruck is the dalmatian, which is known as the "carriage dog" (the "carriage" is the fire truck). The dalmatian is one of the bravest and easiest dogs to train.

The only reason a dog would be found in a firehouse is as a pet. There are such things as arson dogs but dalmatians are mainly kept around for tradition.