Think your retention and your performance as a track & field athlete performance during a season. You want to train to keep on shape (keeping the set retention, 80-90%). During important competitions, the athlete wants to be at peak performance (max retention<100%). So, the coach can plan all workouts and training sessions (Anki review workload) from a period of time before the competition (exam).
Summarizing, you cannot keep a 100% retention long-term, so you focus in having that in the days it matters the most. Using this strategy, you optimize the maximum score chance in the exam vs the manageability of daily reviews.
In Anki specific terminology, you could build a higher retention short-term by filtering decks, decreasing the maximum card interval, etc. Later on, if you still want to remember long-term what you needed for the exam you can change back the settings for that.