Ive been told and i believe that mustard oil brings out a lot more flavour from your spices when you fry them.
I don't think mustard oil brings out the flavour in spices so much as actually adds flavour to the ingredients as it is a very strong oil. It is not a "neutral" oil like say, sunflower oil, but imparts its own flavour like, say olive oil.
Harvey Day recommends the use of mustard oil in cooking curries and he his books were the first I used to cook curries way back in the 1970s. However he wrote his books in the 50s and 80s and they are not representative of current custom in cooking curries and certainly a fair bit removed from BIR cuisine.