This pleated smock is called a rochet. Still, it probably wasn't particularly intended to be flattering. Photo by .
Some pleats are genuinely useful in allowing freedom of movement:
But many are simply for decoration:
Collection of the Metropolitan Museum of Art. Egyptian, about the year zero.
and all are a pain to iron:
portrait by Benjamin Greenleaf
It's an odd thing, fashion.
Still, pleats mean you can fold things:
or unfold them, for that matter, a trick that Nature discovered millions of years ago:
pleated inkcap, Parasola plicatilis. Photo by AJC1 https://www.flickr.com/people/47353092@N00
photo of horse chestnut leaves by Andrew Hill
Then you can find pleats on lampshades, and furniture, and pie-liners, and probably other places, too.
What's the purpose of the first one you find?
Word To Use Today: pleat. This word is basically the same word as plait and ply (as in plywood). It comes from the Old French pleit, from the Latin plicāre, to fold.