All my designer bags are in their dustbags and even some in their box. If its non designer bag but it is made of leather, I keep it in a dustbag as well.
If I buy a preloved designer bag without one, I will look for the same bag online to find out how the original dustbag looks like, and then I will try to buy the exact same on ebay or second hand apps. I know this may sound a little bit obsessive, cant help it
I really think they help preserve the bag and if you ever think of selling your bag, having a matching dustbag adds a bit of value.