I agree but the key is to get test results quickly. A five day delay in getting test results (which is common) seems to imply that an infectious patient may have already done their worst spreading by the time they start quarantining.
So it's not pointless in that sense. You know who might have been exposed.
It's also not pointless for monitoring spread.
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.
Quarantine is mandatory if you had exposure or returned from abroad.