The forest was the home of the snakes, and it was the piece of land that Dhritarashtra had given to the Pandavas. It had to be cleared and made arable.
King Svetaketu (sometimes called Svetaki) conducted a twelve year fire sacrifice, where clarified butter was offered.
After the twelve years Agni (the fire) absolutely needed something else to eat (burn), he wanted to eat (burn) the Khandava forest. Arjuna and Krishna made this possible and the Pandavas could build their capital, Indraprastha, on it.