One of the biggest challenges to containing the spread of COVID-19 is the fact that people can be infectious and spreading the disease before they even know they have it. Because it affects everyone so differently, someone who never develops any symptoms at all can unknowingly spread it to someone who will be absolutely devastated by it. That's why testing is so important.
While testing for the disease has been ramped up, many nations still have trouble testing all the people who need to be tested, due largely to supply issues. As a result, usually only those displaying symptoms can receive a test. Even then, the results can take 24 hours for patients to hear, a crucial period of time when a person carrying the virus can be at their most infectious.