In the UK, if you have a test, you are required to quarantine until you get the results. Is it not the case elsewhere??
Quarantine is mandatory if you had exposure or returned from abroad.
Though, it would very logical as a precaution.
I know people in Poland who avoided getting a COVID19 test because they knew they would have to quarantine if they took the test, and would not have to quarantine if they didn't. That's perverse incentives for ya'.
It's very hard to enforce, of course, but if you get caught, there are (in abstract) severe fines.
So it's not pointless in that sense. You know who might have been exposed.
It's also not pointless for monitoring spread.