So that you can easily stick a foot out the edge for temperature adjustment. Especially with heavy winter duvets, the traditional overhang can be oppressive.
(Related: In Europe it’s common to actually have a personal duvet (twin size) for each of the bed occupants.)
This post explains the two duvet option, which I also think is legit:
Two Duvets, One Bed
When we visited Iceland a while back, I got some of the most sound sleep I've had in recent memory. Instead of a top sheet and one big comforter draped over the bed, Icelandic double beds are topped with two fluffy down duvets (untucked) and no top sheet at all.
Here's a nice synthetic duvet (350 gsm fill)
(maybe better for allergies? — I've heard mixed opinions here because on the one hand, bird feathers seem like they would be bad for allergies, but on the other hand, a good down comforter should have a very tight weave so that none of the feather bits can escape, and that might act roughly like a dust mite cover — where there isn't a natural reason for a synthetic duvet to need such a tightly woven cover)
And here's one that's slightly thinner (250 gsm fill) and so maybe better for summer and/or warmer climates?
Note: Duvet covers are weirdly expensive. I don't really understand why — shouldn't it be about twice the cost of a sheet? I really love the feel and durability of the percale options at LLBean and I like that they have ties in the corners to connect with the loops on the duvet, but I can't resist the diversity of prints (and the lower price tag) of the options at Ikea even though they're typically not as soft. (and they'll ship for $6, so you don't even need a nearby store)