I think they are used to an inhumane extent in some schools and some teachers are overly reliant on them, in the absence of better behaviour management strategies.
My friend's very compliant, hard-working dd was isolated all day because the (fairly subtle) red dip-dye hadn't washed out of her hair properly at the end of the school holidays, so she still had a couple of inches of discolouration on her hair on the first day of term
Other children isolated because the school changed the shoe policy and parents didn't want to replace perfectly good shoes before they had worn out
My 13yo was given 24 hours isolation for clicking his pen while the teacher was talking - I would have thought a verbal reprimand would have been appropriate first, not "straight to isolation"
Our school is a "Ready To Learn" school which is a whole circle of hell in itself.
And yes, isolation is full of kids with ASD and other additional needs ho can't cope with the totalitarian regime the new management (I won't say head teacher, because he isn't a teacher, he's just a manager) has introduced as an Elastoplast solution to discipline and social problems in the school and surrounding community.
Mental health problems among the students are rife and soaring. Resources to support them are lamentable. The usual destination of children who are struggling to cope, particularly if they are boys, is - yep, you guessed it - isolation. Kids can be in there for weeks. Their relationships with staff and peers deteriorate, their behaviour escalates because they are angry and miserable, which leads to more isolation.
There's nothing else - they keep getting isolated until somebody finally calls it quits and they're expelled.
In many schools (including ours) teachers frequently do NOT send down work. Students either sleep, stare at the wall, are told to read something from their bag, or given the same worksheets more than once. The isolation room is supervised by a Behaviour Manager in a high-vis vest - that's her only job, she's not a teacher. She goes around school with a walkie-talkie wrangling kids to the isolation room when teachers need her.
I think in twenty years' time we will look back on this practice with the same horror and contempt with which we remember the Pindown scandal. At least I hope we will.