One-piece footed pajamas are obviously warm and practical, the same reason they are great for kids. I wouldn’t wear any of these myself, because they would be horribly itchy and uncomfortable since they are polyester, but if they were a nice cotton French terry and reasonably priced, I’d buy some.
After reading the reviews for these at Amazon, I’m tempted. Comfortable, warm- what’s not to like? Inconvenient for going to the bathroom, but that’s not a huge part of my personal schedule (yet).
No drafts
Do a Google search for union suit or long johns or long underwear and you’ll find a lot of choices that don’t have to be dressed as an action hero. Quite comfy too! 
Pajamagram has sold these for years in adult sizes. Not a new thing.
Is this a trick question? Because they want to. What’s it to ya?
Ok, I can’t honestly think of a non-fetish reason other than perhaps there could be some developmentally disabled adults who request them from their care givers. (…and yes, that can happen. Its a hard situation, but people deal as best they can.)
As for fetish reasons, if Rule 34 has taught us anything its that what people like can be different from what you might expect, but its their bed & their PJs.
I bought a couple for my mother when she was terminally ill with cancer. One of her recurring issues was poor circulation. No matter how warm the temperature, she would be ice cold. One of the hospice nurses suggested these for us and they gave my mother quite a bit of relief. Plus, they were very plush, soft and comfortable for her at a time when comfort was hard to come by. Hers had snaps at the crotch that could be undone for bathroom needs without necessitating her removing the entire outfit. The outfits had feet attachments that also snapped and could be removed, if desired.
So there you go…an adult use with no fetish overtones. 
Yeah, once you pass a certain age, you know, the one where you have to get up 3 times every night, the onsie loses its appeal.
Speaking of actual onesies, did anyone else wear adult sized onesies in the mid-90s? They called them bodysuits but really they were just onesies in adult sizes with stretchier fabric.
As long as you don’t look like a pink nightmare in one.
There is bit of history on the adult onesie here:
but before you knock it you should try it. Fun, comfy, warm, practical- what is not to like?
She grows…Six year olds no longer fit in the jammies they wore when they were five.
(Me, I donate them gently used after a year or two. Except my favorites, those I still wear until they wear out).
As for adult onesies, there is the Runsie.
Ha, just found this thread and I am reading it in my beloved one-piece tiger pajamas. I would wear them (it?) 24/7 for the rest of the winter if I could get away with it. They are comfortable and cheerful, and I never have to worry about only being able to find the top or the bottom. I prefer no feet because they can pull in certain positions, which is why my actual footie pajamas (fox and sock monkey) have been neglected lately. The only downside is the inconvenience when going to the bathroom.
I bought myself some footed pajamas on a whim and I LOVE them - we keep the thermostat turned down and sometimes I get so cold.
However.
The ones from Target do not have a flap or anything.
Consider how you are going to NOT enjoy that cold bathroom visit.
Thank you, I couldn’t for the life of me figure out why an adult would want to wear a onesie 
A few years back I lived in a house which, while lovely, was no better than a plank house when it came to keeping out the cold. Literally, I had to move my pillow to the foot of the bed because with my head near the wall my hair froze. In Virginia.
When you add in the Celtling, who likes to sleep in Mommy’s bed, and is an inveterate cover thief, I needed some heavy duty PJ’s. I went out on the web searching for thick flannel jammies and the ad bots caught on, and sent me an e-mail for the “Forever Lazy.” Two pair for $12, free shipping.
They are absolutely the best, warmest jammies I’ve ever owned. They are synthetic though, so I usually kept a cotton t-shirt and thermals underneath them. Best of all, they have a really big zippable hiney hatch, more than large enough for clean and comfortable toileting.
I haven’t needed them the last couple of years, as this place is small and easy to heat. But I wouldn’t go camping without them.
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Now you have me thinking of bodysuits (not sure if that’s their real name). I had a couple of those in highschool. Never came untucked or bunched and were not uncomfortable. Haven’t thought of them in years.
Heck, I wore those in the late 70s. Of course they were polyester and zipped up the back of the neck. I remember a rollerskating party in seventh grade…