That strikes me as a fairly modern thing. I imagine that weevils and/or rocks in flour was a common issue just a few generations ago (or, in many countries, to this day).
It can also remove clumps that can form in wetter climates, or from flour of lower quality. During the pandemic I got a big ol bag of flour that was loaded with clumps, and I had to gradually sift all 50lb of it. So, I guess it can still be totally useful in modern times if you have bad purchasing and storage options.