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. I’ve seen it referred to as the “Scandinavian sleep method.”)
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.
But go ahead and measure: Ikea duvet covers have a standard 86” length across all three sizes—that means that if you’re someone who likes to pull the covers over your head and/or tuck your feet like the bottom of a sleeping bag, you may need greater width in order to be able to run your setup at a diagonal, even if a twin size would otherwise work for you personally or for your full-size bed.
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)