Why give a dehydrated dog 0.18% sodium chloride in 4% dextrose? It has hyperthermia and is dehydrated?

it's been rounding up stock all day and is dehydrated and has signs of hyperthermia. Already know it replaces primary water deficits and is a good short term maintenance fluid but that's not specific enough according to the lecturer (yet it is for other students! WTF?)