Why would the tests be needed?

If you're not a sociopath and care about the fact that other people exist.

Not diagnosed = not quarantined. People with generic flu symptoms are not quarantined in most countries. Even if your own treatment is the same, lack of a definitive diagnosis means lack of precautions and more likely transmission to other people.

The statistics matter too. They help health systems and governments coordinate resources and responses and track the spread of the disease across different regions. If they didn't test, we wouldn't know there were cases outside China, Italy, South Korea and Iran and infected people would keep spreading it.