There are a lot of really insightful answers, often dealing with questions of authenticity, mission, where the individual derives enjoyment. I don't have a good, complete, perfect answer.
But I did discover my own, personal derivation.
A costumer doesn't make clothing choices based on the premise, "But I just need something to throw on in the morning when I go to the portajon!"
I realized I'd hit a point where I didn't have such a garment--all of my clothing needs to be worn over stays, pinned perfectly in place. I've had shortgowns in the past, but they've either been given away or are in the "spare things or scrap things" pile.
Of course, full disclosure: Prints and paintings depicting women wearing bedgowns and shortgowns often lead one to believe that they are, in fact, wearing stays underneath these garments. Which makes sense--stays are like basic underwear. But the loose cut and forgiving fit also suggests that maybe they were intended to be thrown on as a kind of pre-stays wrapper in the morning or post-stays loungewear at night. Speculation on my part, yes--but it's how I find myself using a roomy garment!
Is this lady wearing stays beneath her shortgown? Hard to tell--but I love that scowly expression.
"You kids get off my lawn!"
I've used this pattern before--in fact, it was one of my earliest 18th century projects. The pattern is something like this one:
from the Mara Riley website, except it's a little longer and the pleats in the back are a little different.
Another place you can find a similar pattern is in the book Fitting and Proper. Costume Close-Up has a shortgown pattern that, instead of using pleats to fit the back, uses a drawstring.
I have no idea where the original of this pattern came from--it was passed around the ladies of our regiment years ago, and I suspect that the originator drafted (or Frankensteined) the first pattern herself. I copied my friend's copy onto the wrapping paper affectionately referred to as the BabyBabyBaby paper:
This time around, of course, I had my own BabyBabyBaby to contend with while laying out the project.
She likes fabric. A lot.
It's the easiest pattern in the world to cut out--you lay the center edge (the one with the neckhole in it) on the fold, pin 'er down, and cut 'er out. The center front will need to be cut to form the opening.
And when it's cut out and unfolded, it looks like this:
This is the lining--basic white linen. See how the center front has been cut so it opens in the front? That's the trickiest part. Easy-peasy pattern.
Next time--the assembly of the shortgown, my choice of fabric, and the Great Authenticity Decision plays out in this project.