After creation in Celestial/Upper worlds is over, Brahma was asked to start creation on Earth.
Brahma could not find Earth. It was abducted by Hiranyaksha. Vishnu took incarnation as Varaha swamy, killed Hiranyaksha and kept Earth in its original position in the orbit.
Brahma could not decide how automatic continuous creation for crores of years is to be arranged. He sat in meditation. Om, pranavam, came from his mouth involuntarily and from that Rudra appeared before Brahma.
Brahma told the problem.
Ridra took the form of Arthanarishwara. Brahma came to know that female and male are necessary for automatic continuous creation.
Arthanarishwara is symbol of creation. Without Sakti, Parvati, even Siva is inert.
Om Namah Shiivay