Places are limited, so course tutors need to find a way to decide who gets a place and who doesn't. (For example, courses in motor mechanics probably get ten or twenty times as many applications as there are places).
Courses in certain subjects have obvious links to school work. e.g.the training to become a qualified electrician involves quite a bit of maths and physics, so tutors might be looking for a minimum C grade, at GCSE, in maths and a science subject in order to select the most suitable candidates.
Similarly, those who've shown their talents in CDT might get preferential treatment when applying for carpentry courses.
Even when there's no obvious link between school work and the content of an FE course, tutors may see good exam results as proof of:
(a) an ability to learn things fairly easily ; and
(b) a willingness to study hard.