At my school, they just affected AS predictions and that's about it. I'm not sure if they focus on particular grades or not. Either way, it won't make a massive difference. It's just a guide.
Your A2 predictions are the most important thing, and that'll come from your AS results. Depending on how you do in those, they tend to predict the same grade or one grade higher (for example, if you get ABBB, they might predict A*AA or AAB etc.)
Mine weren't based on GCSE results, my AS predictions were based on my progress through the year. To be honest your AS predictions aren't really that important as they are not published, your A Level predictions are the ones that go on your UCAS, and these are usually based on your AS results - so get revising!