It's one of the very few ways you can obtain fragments without using seals, another way would be Survival Trial but that's just random and you don't necessarily get the fragments of the character you wanted.

I'd suggest keep sweeping, it's random but sometimes you get 5 fragments in 3 sweeps believe it or not, and other times you get just 1. Don't sweat it and keep trying so you can get all your favorite characters upgraded :P