Shiva is often considered as the God of destruction. For destruction, one requires Shakti (power), which happens to be His consort. It is a general principle that the power and the possessor of the power cannot be separated.
For instance, a sword and the quality of ‘sharpness’ cannot be separated. They go hand in hand. [‘Shakti-shaktimato: abheda:’] Hence Lord Shiva and Parvathi are depicted as inseparable, in the form of ‘Ardhanareeshwara’.
For the Lord that preserves and maintains the Universe, all the living beings expect grace and compassion. Compassion springs from the heart. [That is why when someone is rude, we call him heartless]. Vishnu, the Lord of Preservation, has His consort, the Mother, who is the epitome of compassion, verily in His heart. [We see this in Sri Venkateswara Perumal who has His consorts on His chest]
Similarly, ‘Vani’ or Saraswati, the Goddess of knowledge is said to reside verily in the tongue of the Creator Brahma.