There are several different specialized fields that police and military units use several different specialized breeds of dogs for. Each of the different K-9 units trains dogs with a specific task in mind. Therefore different breeds are used for different tasks based on their inherent instincts and natural and physical abilities. Some breeds have a better sense of direction that others, so they would be suitable for tunnel and cave-in type rescue work. Some of the smaller Dogos would probably be great at this because there is a friendly face to lick at the end of a tunnel.
Some breeds have a 'colder' nose that others, meaning a better nose for weaker scents.
They are more suitable for trailing up an older track. Like Bloodhounds for example. They are not used to actually catch the bad guy, just track him down through the swamps or other such places with a difficult nature, too cold for most dogs noses. Where as a 'hot' nosed dog is used to pick up and react on a much fresher track and therefore a different breed is used for that task. They are separate units and the dogs are trained for different scents.