well in the past you needed to pass your GCSE in order to do the A level in it (OK in some cases they asked for a higher grade, but the principle is same)
Also for many Further Ed /university courses it is a requirement for you to have grade C in Maths and English.
Our local sixth form college has a group every year retaking their Maths and English.
eg to be a teacher, you need 3 A levels plus GCSE Maths and English at grade C. So if you take you GCSEs now, get a grade 4, don't retake, do your A levels and apply to teaching, you may suddenly find they are asking for grade 5 in GCSE Maths and English, and you didn't retake and up your grade, and you only have a grade 4.
So, that disctinction between and 4 and 5 could be crucial.
Given all that I have read about the new Maths, it seems as if it is really hard anyway, and so many kids may find themselves in this position
So this will have an effect for kids sitting this year.