Open Clothes
Just look at open clothing; it's one of those designs that has been done for a long time, but there's a good reason for that. You'll see how, with an open shirt or an off-the-shoulder jacket, the character suddenly transforms from stiff to relaxed. It's that candid look that fans seem to enjoy for their characters. Artists use this to flaunt their good drawing skills, especially on the collarbone or with the slight blur effect of the clothing in the back.
But, honestly, the draw of this design seems to stem from the specific mood that's being portrayed. It could be a cozy morning scene or an action scene with rugged features, but you'll see lots of orange or blue shades emphasizing the character's shape. And, of course, there's the fact that this gives the character more personality, which cannot be said of their standard outfits.
