Why is Mary Important?

Introduction
Mary's future importance in the story is one of the main mysteries in the novel.

Set-up
The mystery surrounding Mary has been set up since the Prologue. All that is told is that she has an "important role to play" in the future. Throughout the novel, it becomes apparent that Mary has a natural talent with magic that borders on impossible given how long she has known she is a Witch.

Answer
This mysteries is still not solved as of the ending of the novel.