Is the real reason for these rooms that schools have to take on pupils they can't cope with but there isn't any other sort of provision for them?
No, a lot is linked to a change in parental attitudes to behaviour and rules, as well as society attitudes to schools and teachers.
When I was at school, I'd be bothered about a teacher calling my Mum. I'd know that if school had to speak to my Mum, Mum's response would be to ask me what I was playing at and to get on with my work. Now there is a sizable minority of parents who actively undermine the school, think that their child doesn't have to follow any rule that they don't like, and will sit in front of their child and be aggressive, confrontational and outright lie to staff in order to defend their child.
When I was at school we had a simple uniform that was strictly followed. Now schools have to deal with situations where simple things like "no trainers" mean some parents go and spend over £100 on a pair of branded trainers, and then come into school complaining that they're not made of money and can't afford more shoes because school have said that "no trainers" means no trainers. Or they'll tell you their children's skirt was definitely knee length when they left the house in a morning (knee length wasn't our rule BTW because we appreciate girls have different bodies and different leg lengths, we just don't want to see pupils' underwear) when you are all sitting in a room with the student and the skirt quite clearly was never mid thigh, let alone knee length. Why the parent lied to our faces in front of the child is beyond me, but as you can imagine the student knew their parents would always back them. It will comr as no surprise that those students found themselves in isolation at times because parents have told them they don't have to follow rules.
Then add in the minimising of disruptive behaviour (eg my child was given a detention for asking a question vs my child was given a detention for repeatedly talking when told not to, the teacher said if anyone needs help to raise their hand, their child turned round to talk again and after 4 warnings they were removed from class and given a detention). When this happens several times in a day or a pattern emerges, the student has time in isolation because several hundred other children's learning come before one child's desire to disrupt and talk to their friends.
Then add in parents who claim schools can't give their child a detention, they don't consent and their child doesn't have to attend. The child doesn't attend so school move through the policy, and the child goes to isolation.
If a student is in isolation then they are in school, are seen by staff who can check welfare, they've got work to complete, and are continuing with their education. Because of some of the parental attitudes above, a fixed term exclusion would mean a trip to the shops, chilling on their xbox etc, or for some students being in potentially dangerous situations or engaging in behaviour that could put them in trouble with the police.
As with any initiative, it can be used effectively, used ineffectively and needs to be reviewed regularly, but it's a much more complex issue than some of the anti-isolation people would like to present.