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?
Using the overnight method for oats is more beneficial for oats that aren't quick oats. For example, steel cut oats can take ~20 minutes in a pressure cooker, then there is still the prep time to add all of your ingredients.
Using the overnight method, you can just dump everything and let it soak overnight. The total amount of work is lower.
The bigger benefit, though, is that you can prepare a breakfast meal the night before or even days in advance, and have a ready to grab meal.
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.
To add on to the answers above, for folks who work in an office, having something that can be easily taken to the office to be eaten at one's desk is a benefit, and that's easier if there's absolutely no morning prep time and you don't have to worry about cooling it in order to transport. You can make a big batch and break it up into small servings, whereas if you did that with normal oatmeal you'd still have to have a way to reheat it to eat. And if you do that and make the oatmeal in the jar or whatever it will be stored in, there's only one thing to clean instead of two (bowl + pot you made it in).
Plus, if you live in a place where it's hot outside at least part of the year, cold breakfast can be nice. At least in the US, the classic cereal + milk breakfast is cold, too, so this can be a nice replacement for that, temp-wise.
Another benefit of soaking is the reduction in phytic acid.
Soaking helps reduce the phytic acid present in oatmeal, which can help make oats easier to digest for some people. Phytic acid in larger quantities can blocks the absorption of certain minerals into the body.
This is partly why people soak beans overnight as well.
Here's some more background:
https://www.bobsredmill.com/blog/healthy-living/5-reasons-try-overnight-oats/ (general benefits of soaking oats, including phytic acid reduction)
https://www.healthline.com/nutrition/phytic-acid-101#in-food (background on phytic acid)