I also am wondering this because I am working on my weight. It went up over the past 10 years, had to purge a lot of the stuff that didn't fit anymore, bought new clothes for the current me and now I am worried if I lose weight that the new wardrobe I bought won't fit anymore!
I am saving the best stuff from my prior wardrobe in case they start to fit in the future, although they may not ever fit again, it's stuff I can't really replace so I'm ok having a few storage bags of items. I did offer them to DD to have or borrow, but she was too busy the last time she came to visit (she was only here for one night).
I did buy a couple of items in a smaller size than I'm wearing right now. One is a Gap barrel jean with a floral print that also had a matching denim jacket, they are already roomy for the size listed. I bought the M but I kept the S in case I lose weight. It was on sale and not that expensive so NBD if I don't use the small. Another was a pair of plain barrel jeans, I bought one size down in case the one I'm wearing becomes too baggy. It was already on sale and bought it from Nordstrom, so I could potentially return if I don't end up using it. It was also not that expensive ($65) so not a big deal if I don't use it. I have the feeling that my overall torso/hip size is not going to shrink even if I lose 10 lb, hopefully it'll just mean my apple/food baby gets less protuberant. The overall style seems to be looser nowadays so I tend to upsize from M to L, hopefully it'll still look OK even if I lose more weight.
I would say if you really love the item and it's unique and you have the space and money, then buy and keep two sizes of your favorites. If you can always get something similar later, then don't worry about buying multiple sizes.