Each problem in the exam is worth some number of points (20), I think that the aim is to ensure that you can't pass by mediocre performance across many problems, that a bare pass indicates that you have retained enough knowledge/ability to get basically correct answers on two problems.
Explicitly the aim is to eliminate students who haven't deeply understood some aspect of the curriculum, so accumulating lots of partial results is exactly what they don't want.
It's worth noting that out-right failure is extremely rare and subject to an appeals process etc. Partly this is because this is a set of exams at the end of each year of instruction with no mechanism for a re-sit, so a student who fails will not graduate (the system isn't totally barbaric, there are mechanisms in place to handle health related concerns etc.).
Explicitly the aim is to eliminate students who haven't deeply understood some aspect of the curriculum, so accumulating lots of partial results is exactly what they don't want.
It's worth noting that out-right failure is extremely rare and subject to an appeals process etc. Partly this is because this is a set of exams at the end of each year of instruction with no mechanism for a re-sit, so a student who fails will not graduate (the system isn't totally barbaric, there are mechanisms in place to handle health related concerns etc.).