I use a room to store gyroids, and another is empty. The last two, I haven't purchased.
The empty room used to store Nintendo items, until I realized that it was pretty pointless - they may not be orderable, but they're also not (what I consider to be) nice looking.
I might eventually put themed rooms of furniture in the Museum, but that's a long way off.
I use them for the same old reason as everybody else which is for storage but I just happened to take a picture of all four recently so I might as well contribute
Flower set and other unorderable items I like
Gracie clothes
Can't see but all my trophies, HRA stuff, random event items go here in display cabinets.
Storage for random items I like that there's no room in my house for
I currently only have a fruit cafe (although I might move that to one of my side characters' homes because she needs a kitchen and it would fit her theme), but I'm planning on adding a library, an RPG-style inn and possibly a gym.
I have one 'Town Name Memory' room, where I place a basket of the town fruit and my tools I don't need and stuff. And clothes I used to wear and stuff that reminds me of good times, lol!
I have one room for storing things I don't have enough storage space for. Just took a few random things and threw them all in a room.
In my main town, one room is clothes. I was hoarding it in preparation for my Fashion Checks, which I passed all of ages and ages ago, and now I just have a room full of clothes I like to look at but never wear because QR codes. .-.
I have one room full of 711 furniture and hybrids. I really need to get all the hybrids out since they take up a lot of space and I have the Beautiful Ordinance now.
Then there's one room full of miscellaneous unorderable items. My wetsuits, Gracie furniture, DLC, etc.
And lastly, I have one entire room full of spare Sloppy Furniture. Like, full full. No space to walk. I have more in my locker that I can't fit. xD; I discovered more in my other town, too. It turns out that when you TT a lot, villagers put a lot of in in Retail...
I have a hard time getting rid of Gyroids due to the whole story about them maybe being past mayors and what not so I've been trying to set them up comfortably in the rooms.
One of my rooms is a Museum display full of every Cyrus customized miniature fossils in large glass displays ... They all just barely fit with room for a desk and bench! Lol
Another room is floral themed, lined with tables and it displays my precious entire set of hybrid seed bags collection. My Flower set is also displayed here.
Another room is full of street pass balloons and gold nuggets ... Eventually will be my themed golden chess set room (my basement is holding some of my golden chess pieces)
The last room is my Bug and Fish tourney room with some rare DLC pieces that I personally love.
BTW ... Is anyone else with me on a wish for a third Museum Floor and more rooms!!??
One is full of all the shells you can find on the beach, along with some music boxes and other beachy stuff. One is going to be a cafe, one will showcase my achievements in Larkspur (trophies, etc.), and one is going to be like a little gift shop I think.