The difference with mocks is that they take place over a much shorter and more intense time period than the actual GCSEs. DS did all his in a two week period straight after Xmas, generally doing two exams a day. At least he had Xmas to revise though, as some schools did them before Xmas.
Some schools make a much bigger deal about mocks than others, eg posting a results letter, etc. DS’ school very much downplayed the results, saying that they were mainly intended as practise only and to highlight any areas they may need to focus in for July. They also marked them very harshly ( in the sense that nobody got a 9) because they didn’t want anyone becoming complacent).
DS had a headache by the last day of his mocks because he’s done 3 exams that day. On the last Physics paper, I think he scored a 5.
In the real thing, he paced himself much better and was hoping for at least 7s, based on most of his mock grades, He got ten grade 9s and most of his school got all 8s and 9s, even though they weren’t expecting this based on their mocks. To be honest, I think DS’ teachers might have assessed him as capable of a 9 grade in most subjects, but would have probably given him 8s at best in Maths, Physics and Biology. He really pulled it out of the bag in the last term and surprised himself and from what I can see, this is very a very common pattern. I do hope teachers factor this in for this year.
Having said this, for children who fall apart under exam conditions, ongoing assessment is no doubt a good thing. And anyone can bomb an exam so I guess this strategy at least protects them against that possibility.