One of the most important things in a witch's arsenal is neither wand, nor athame, nor any physical item. Ritual does not even require any of these things, in truth the most important item is knowledge. A witches Book of Shadows, is where he/she records his/her knowledge, learnings, meditations, and the steps of her/his journey. This great book is akin to a journal and yet so much more for it can be passed thru the years to each new generation of the family filling to overflowing with the knowldege learning and observations for each new witch to study and be guided along thier path.
This makes it not only important to the one that creates it but to all those that will be blessed to read it in the future.