It's for contact tracing. So, the app exchanges code between phones when they are near each other (kind of).

Let's say my phone sends you a code because we were near each other. Then I test positive to covid after that. I can let someone in the health care system go in the "main program" and enter that I was found positive. Everyone that has one of my code in their phone will be alerted that they were near someone who was declared positive.

You can now go to test yourself too. That's faster than having people calling and doing manual contact tracing.

Contact tracing is important to keep the spread of the virus low.