What is the purpose of dragon buffs being permanent?

So you can snowball your lead or if you're behind it'll be a small way of coming back

If they were temporary, the buffs would have to be on the level of Baron or Elder to be worth fighting over. It gives us another objective to control and another path to victory by stacking buffs to enhance our team. If we only had to knock down turrets, the game would get stale.

If they were temporary, they would have to be buffed to a point where you always had to contest them in order to prevent rotations that would cost you many of your own objectives. Otherwise, they would be next to meaningless. It also promotes objective control and rewards smart players that may not be mechanically so great.