Veda Vyasa Maharishi who is the 17th incarnation of Lord Vishnu, divided the one Maha Purana into 18 Puranas to help mankind during the dark age.
All these 18 MahaPuranas are preaching of Suka Maharishi to the Sages who were doing Mahayagnam in the Naimisaranya region to attain the realm of Lord Vishnu.