Often it makes no difference if someone has got one grade or the other. Sometimes though, v.popular universities will sift based partly on GCSE results, when particular courses are v popular. In that scenario, the difference between a 7 and 8 or 9 might make the difference in getting an offer or not. Most of the time, it won’t matter, but at the very top end, it can. When some courses have far far in excess applicants the places, and very large numbers have A* predictions at A Level, the admissions depts need some way to sift applicants, and GCSE grades is a simple and obvious one.
There’s a real difference in appraoch to asking for reviews. In some schools, most parents who have a child who is close to a boundary will be asking for the review. Some have multiple ones done. They really aren’t bothered about spending a couple of hundred quid on it. In other places, parents wouldn’t spend the £11 needed to get a script back, even if it’s suggested the marks are seriously out of kilter with what was expected.