Because the fire flushes out small prey that the bird can catch and eat. This applies to snakes, small animals, and insects, all of which this species of bird and others can feed upon.
I’m in the same boat as you, but in the book Sapiens it’s written that bush fires in Australia are started to force animals out of the forest and to create plains where they can be more effectively hunted in the future. Maybe the birds do this too (or learned it from people?)
You may have conflated a couple of things here that I think might be worthy of clarification.
1. Fires will force animals out of the forest for hunting. It sounds to me that this is the most likely reason for animals learning to spread fire.
2. While I expect sufficiently fierce fires will create plains, much of Australia's native flora is adapted to bush fires, and use it as an opportunity to regenerate. Many plants have seed pods that will only open after being burned. Thus, while much of Australia's bushland is dry, bushland that has been burned recently tends to be quite green and full of life, and many of the trees remain - however much furrier for their fresh growth of leaves.
3. Australian Aborigines (to my best recollection) were known for cycling parts of their territories as part of their nomadic lifestyle, farming somewhere, burning the land as the left, moving on, then returning again later when the land is restored and more plentiful.
