In India, the harvest season begins at this time; and as mother earth is the source of all food, the Mother Goddess is invoked to start the new harvest season and to reactivate the vigor and fertility of the soil. This is done through religious performances and rituals which invoke cosmic forces that are believed to rejuvenate the soil. On the day of Dasha-Hara, statues of the Goddess Durga are submerged in the waters of the river. These statues are made with the clay, and the pooja is performed with turmeric and other pooja items, which are powerful disinfectants and mixed in the river waters. This makes water useful for the farmers and yields better crops.
Chatrapati Shivaji Maharaj, the founder of the Hindawi (Hindu) Swarajya - Maratha Empire used to always worship Lord Shiva & Goddess Durga in the form of goddess Bhawani before any military expedition. Goddess Bhavani had blessed Shivaji Maharaj with her own sword called "Bhavani Talwar" on this blessed day.
Dasha-Hara is the festival of Victory of Good over Evil. Buses, trucks and machines in factories are all decorated and as Dasha-Hara is also treated as Vishwakarma Divas - the National Labor Day of India. Veda Vyasa is considered as the foremost Guru and Vijayadashami is also celebrated as Vyasa puja. Shastra pooja (great worship of weapons) is also especially carried out on this day. The weapons (Sastra or Astra) used by Goddess Durga are worshipped on this day.
some of the main causes of this great day are-
Victory of prabhu Ramchandra over Ravana
Victory of Durga Mata over Mahishasur