“So is number of infected per 100,000.”
I’m not saying that number isn’t important and you often see tracked in the media. But you’re only reporting results of people actually tested, so it’s less useful in trying to get an idea of how widespread the virus is if the number of tests vary.
Take extreme examples where testing rates are more useful than positives per population:
Area A: 100,000 random tests with 1,000 positives
Area B: 1,000 random tests with 1,000 positives
Or the following:
Day one: 10,000 random tests with 100 positive.
Day two: 5,000 random tests with 50 positive.
In these examples, if you looked only at positive tests per population, you’d get a very distorted view of what’s actually happening.