Plenty of spices distribute their flavors better in oils/fats than in water.
Take some szechuan peppercorns and boil them in a soup and see if you feel the ma numbing sensation. Then saute them in oil for a few minutes and then add the soup, and notice the difference.
Take some szechuan peppercorns and boil them in a soup and see if you feel the ma numbing sensation. Then saute them in oil for a few minutes and then add the soup, and notice the difference.
It's the same with lots of spices.