In addition to the other excellent answers, there will always be some students who will assume that the marking scheme for the final exam is similar to the one for the mock exam no matter how often you told them that this is not the case.
But since the questions in the final exam will obviously be different, the marking scheme there will most likely be different, as different errors can be made by the students and must be taken into account in the scheme. Also, some parts of some questions may be trivial in the final exam, and you may want to assign a fewer fraction of the points for those.
So you will have to convince some students who will try to argue that they should get more points using old marking scheme and claiming that it would be unfair if the marking schemes differ substantially.
As a lecturer, you don't want to spend you time with such discussions.