Arlen Holder presented the following explanation :
On Wed, 16 Sep 2020 16:21:07 +1000, wrote:

It will tell you that you have been that close to that individual
for that long if they later turn out to be positive.

And what if that asymptomatic person never got tested?

Maybe he doesn't have the disruptive disease, yet passed the virus to
someone else who went on to be tested and traced back to you and others
through him despite himself having still no symptoms.

Scenario - an asymptomatic "case" is in front of you at the grocery
checkout. He infects you and others and then goes home to infect his
friends and family. His parents get sick quickly and the app helps to
show that he possibly infected many at the store at that time and for
some hours afterward - you get an alert since he has now been tagged
and you were in close enough proximity for a long enough time to be at
risk.

Sure, like everything else, it is not perfect, but that's no reason to
discourage people from using such a measure if they want to.