Your point would had validity and logic if we had an effective test and tracing system. As we don't, then quarantining is the only effective way (if we had complete compliance) to mitigate further spread. It's a wrong comparison to say that because the net countries have the same amount of infection then it's no different and therefore no quarantine should be needed. Just because it's here already, means it doesn't make sense to compound the infection rate by adding more to it.
For example, if we take Dorset as a LA with a low case count (8.2 per 100k of people), and someone comes back from Spain without quarantining. Has the virus, and then spreads. Dorset then have to try to contain an outbreak. Could be mitigated with an effective quarantine, or test and trace (with the theory of being able to trace contacts and then tell them to isolate), and has a chance of keeping the case rate low.
The quarantine logic is as more down to controlling local outbreaks, which if uncontrolled, lead into a snowball effect into other areas. This was the issue that was in Bolton and has caused a ripple effect into other areas of the North West (Liverpool, Halton, St Helens specifically).