They provide a guide to a child's ability, very necessary when the child may have other problems. The state primary school my children attended dismissed a friend's child as 'just not very clever' because of his reading difficulties. An educational assessment, including IQ tests not only showed that he was actually dyslexic, but that far from being not very clever, he was actually very clever. Something anyone who knew him well was very aware of. But teachers do not listen to parents and their friends, but an IQ test convinces them.
Nor is his case, the only one, their use occurs regularly in court cases when it can be shown that a defendant was of limited intelligence and unable to fully foresee or understand the results of actions he took.
I used to work with gifted and talented children and several came our way when they had been referred for an educational assessment because it was thought they had learning difficulties. IT turned out that they were exceptionally bright and bored rigid at school so had just zoned out completely and completely dissociated themselves from everything going on in the classroom, giving an impression of learning difficulties. I could go one but I won't.
I think this obsession with IQ tests not being indicative of anything, not reliable etc etc. Is all part of the anti intellectualist slant of British Society. I have never heard any one make these criticisms of EQ or measures of creativity or that holiest of the holiet statistics, BMI, which are just as approximate as IQ.