I don't even understand why that option is there?
Personally, I would like to be able to buy fuel by the tonne in the same way as we buy commodities or limpets. If you're long-range smuggling and you've fitted an extra fuel tank, it can negatively impact your jump range if you have to take on more fuel than you need by always filling the tank. Pilots in the (other) real world have that ability; it would be useful if we could too.