That is a reasonable query. My understanding is that the purpose is to judge your performance as to whether you are suitable to the MPL or whether they would feel you may be better off on the fatpl.
So they essentially gather the group they think will be best suited to the MPL (route 1), then those who they think may suit the MPL but they'd like to test in action to find out (route 2), then those who they feel are best off on the fatpl and most suited to that (route 3).
I could be wrong but this is how it appears to be based on what i've been told.