What the breed was made to do
The Australian Cattle Dog was developed in Australia in the 1800s. Ranchers needed a tough dog that could move half wild cattle over long distances in heat and rough land. A cattle man named Thomas Hall is often credited with starting the line in the 1840s, in New South Wales. The story goes that Hall crossed blue speckled collies from Britain with the native dingo, and later breeders kept shaping a hardy, quiet, heel biting cattle dog. Parts of that story are told with some caution, so treat the details as the popular account.
The job explains much of what you see. A cattle dog moved large animals by getting behind them and nipping at the heels. They had to be tough, tireless, and quick to react. They worked with a person, but also on their own at distance. Read our Australian Cattle Dog guide for more.
That does not mean every Heeler is the same. Individual dogs vary a great deal. But many show some mix of intensity, stamina, alertness, and a close bond with one or two people.