The Divine Mother always exists in Her Universal and Transcendent, JagatJanani and ParaShakti aspects, but She incarnates in a human body with a specific purpose and for a special work when it could not be done otherwise. It is the same as with the Avatars of the Ishwara wherein each Avatar takes up a special and specific work for which he alone has the mandate.
The Yoga of transformation can only be undertaken and fulfilled by the Mother. In this way and for this reason the Avatar, even though an aspect of the Ishwara, is in certain ways greater in terms of the specific work that he comes to fulfil.
The Divine Mother is always available to help, to bless, to heal, to succour or to give freedom, peace, wisdom, love and bliss as well as countless other boons and gifts which equally can be granted by the incarnate Mother in human form.