in some subjects like maths a level, you need a certain amount of UMS to get an A* or A overall. if i get 100 ums in the easy modules like c1 and c2, i can do much worse in the other modules. plus more ums = being better than other people, if two people apply to uni with the same grades, theyre gonna pick the person with higher ums.
i would kill myself if i didnt get 100ums in my AS maths modules lol