It is to make neat folding trousers. You fold up your pants legs with this underneath.
Called blousing straps.
There's also blousing straps that go around the calf instead. Black Elastic Blousing Garters Set https://www.amazon.com/dp/B001022G2C/ref=cm_sw_r_cp_api_VsQ.xbJQY03XS
We wear them in the military to keep our pants in place under our combat boots.