According to the Mill City Museum's website, you're allowed to bring in small bags and backpacks, which is pretty cool. This means you can still carry all your essentials, like your phone, wallet, and snacks, without having to worry about leaving them behind. It's kind of like going to a music festival, where you want to be prepared but also don't want to overdo it.
Plan Your Visit | Minnesota Historical Society
But, if you have a large bag or luggage, you might need to leave it behind or check it in, which is understandable. You wouldn't want to be lugging around a huge suitcase while trying to enjoy the exhibits, that would be like trying to hike with a heavy backpack - not the most fun experience. The museum wants to make sure everyone has a great time, without any unnecessary hassle.