1. Durga kills the asuras

    (For Durga-puja; from Srila Prabhupada's purport to  SB 4.24.19)

    Lord Siva is always accompanied by his material energy (saktya ghoraya). Material energy — goddess Durga, or goddess Kali — is always under his control. Goddess Kali and Durga serve him by killing all the asuras, or demons. Sometimes Kali becomes so infuriated that she indiscriminately kills all kinds of asuras. There is a popular picture of goddess Kali in which she wears a garland composed of the heads of the asuras and holds in her left hand a captured head and in her right hand a great khadga, or chopper, for killing asuras. Great wars are symbolic representations of Kali's devastation of the asuras and are actually conducted by the goddess Kali. srsti-sthiti-pralaya-sadhana-saktir eka
    (Bs. 5.44)

    Asuras try to pacify the goddess Kali, or Durga, by worshiping her in material opulence, but when the asuras become too intolerable, goddess Kali does not discriminate in killing them wholesale. Asuras do not know the secret of the energy of Lord Siva, and they prefer to worship goddess Kali or Durga or Lord Siva for material benefit. Due to their demoniac character, they are reluctant to surrender to Lord Krsna, as indicated by Bhagavad-gita (7.15):

    na mam duskrtino mudhah
    prapadyante naradhamah
    mayayapahrta-jnana
    asuram bhavam asritah

    Lord Siva's duty is very dangerous because he has to employ the energy of goddess Kali (or Durga). In another popular picture the goddess Kali is sometimes seen standing on the prostrate body of Lord Siva, which indicates that sometimes Lord Siva has to fall down flat in order to stop goddess Kali from killing the asuras. Since Lord Siva controls the great material energy (goddess Durga), worshipers of Lord Siva attain very opulent positions within this material world…

    Since a devotee has nothing to do with any kind of material possession, he does not come under the control of goddess Kali, or the goddess Durga. Lord Siva is also in charge of the tamo-guna, or the mode of ignorance in this material world. His potency, the goddess Durga, is described as keeping all living entities in the darkness of ignorance (ya devi sarva-bhutesu nidra-rupam samsthita). Both Lord Brahma and Lord Siva are incarnations of Lord Visnu, but Lord Brahma is in charge of the creation whereas Lord Siva is in charge of the destruction, which he carries out with the help of his material energy, goddess Kali, or goddess Durga…