I'll bet you got this idea from an Internet site proclaiming "the miraculous benefits of onion skins" or something similar...right? Good for treating athlete's foot, premature baldness and flu symptoms; excellent hair thickener, skin treatment...but wait, there's more! Good for plants!
Until composted, they are kitchen waste sitting in your flower pots. They probably won't hurt unless you pile in so many they start using up nitrogen, and aren't nearly as messy as the other Internet wonder, banana peels. Given how insubstantial onion skins are, I can't imagine that even when composted they'd amount to much of anything, useful or otherwise, but hey, no harm trying!
My mother used onion skins to dye her hand-spun wool yarn during her textiles stage. Produced a pale brown hue. Another "miraculous" use if you're interested.