The primary advantage I hear people talk about is that overnight oats take "little prep time", but isn't that true of oatmeal anyway? Is it just a fad because it's something different and you can use a mason jar? Or is there a true reason or benefit to overnight oats compared to traditional cooking methods, particularly quick oats? 
 They taste different. Some people like the taste of "overnight oats" better than regular oats, and some people absolutely hate them.  "Put a bunch of stuff in a jar and leave it" is simpler, for some, than cooking oats on a stove (or even a microwave).  Overnight oats prep happens at night, at which time many people are less rushed than in the morning, when oatmeal is typically eaten. I, for one, would not have time to cook oatmeal in the morning, but I definitely have time to prepare overnight oats the night before and throw it in my bag before leaving to work.  Not everyone has a way of cooking oats at the place where they usually eat breakfast. I suppose you can eat traditional oatmeal cold, but likely many people would prefer overnight oats to traditional oats for eating cold.   