Lord Shiva and Devi Parvati are the Masters of the Material Universe. Lord Shiva of the Material Universe is called Shankara and Devi Parvati is called Devi Durga, in the material universe. Lord Shiva also represents Guru. Without Guru, higher attainments are impossible. So, without Lord Shiva's consent nobody can rise to spiritual heights. Just as the Guru is not optional for spiritual growth, Worship of Lord Shiva is not optional if at all we want to rise spiritually.