The point is that the LFTs may be slightly useful to pick up unsuspected cases.
So e.g. a weekly testing programme that tests everyone may pick up some extra positives - as long as those who test negative behave exactly as they would had they not been tested at all, with all current precautions scrupulously observed.
They may also be useful to test a wider population than might isolate e.g. if a school is sending home close contacts and there is a group of 'more distant wider contacts (e.g. if everyone in a class with a child who tests positive isolates, but those who e.g. were in the canteen at the same time has a LFT, or perhaps in schools where they are sending home the '2 metre' contacts only, it would be slightly less foolish to also LFT everyone else in the classroom over a period of time, which might also inform a change in policy).
It was always obvious that they should not be used instead of isolation, and I am glad that the Government should no longer be able to worm their way out of this (I had already written to my Conservative MP, who was sufficiently concerned to be consulting ministerial colleagues).
i don't think this is the end of the testing in schools plan, but it should only ever be used as a routine regular screen (with suitable information about what a negative means) or in addition to isolation.