It’s for extinguishing oil well fires, owned and developed by MOL, the Hungarian oil conglomerate. It was used in Kuwait after the Kuwait-Iraq war, here is a video of it in action: https://www.youtube.com/watch?v=uyGDxglTVgA
It’s still in use and maintained, the base was replaced with newer tank recently, as it was hard to find replacement parts for such an old model.
Video from some years ago, timestamped, there is a lot of Hungarian narration before that point: https://youtu.be/YYF8YQ7pLng?feature=shared&t=494
Good article in English about this beast, with a lot more images: https://www.autoevolution.com/news/a-firefighter-s-dream-meet-big-wind-a-tank-with-jet-engines-that-s-a-fire-truck-185011.html#
I was going to correct you where you say "3000 litres of jet fuel as I was thinking water would be better for spraying! Then I remembered about the jet engines … 😅
Actually water is used only for cooling the surroundings. The fire is extinguished by the air current, the same principle as you blow a candle, but at a different scale, it would work without water, but the heat could reignite the gas.
Water is piped from local reservoirs. This was a problem in Kuwait, where oil wells are in the desert. The advantage of this design was it uses much less water than other fire engines.