The government has previously bought large supplies of Covid tests with no publicly available data on whether they worked. A test result that is available within minutes is far more valuable than one that takes at least a day. Having few false negatives is more important when you’re trying to diagnose sick patients who need Covid treatments. False positive tests may also result in unnecessary self-isolation until a second test result is available. Imagine that a Covid test generates one false positive reading for every thousand people.

Source: The Guardian September 11, 2020 13:52 UTC