Is this a thing, as in do you see it in the wild, as opposed to just a skirt or something? First of all, look at this guy:
He is wearing crotchless leggings, so the breechcloth is hardly superfluous. But then we have
Now a man wearing dinner clothing might wear a cummerbund which, besides providing a pocket, serves to conceal the transition between the shirt and the waistband. The woman in the photo, on the other hand, leaving aside that she is dressed casually, clearly has the edge of her shirt visible.
The loincloth in the drawing is slung over a girdle strap. Loincloths could also do without this by starting with the front flap, under slinging the groin, then wrapping around the waist and making a back flap
Instead of a loincloth, Europeans opted to fill the gap between hose legs with codpieces; often of elaborate design.
You are probably right in that just the tights without anything to offset them would not look as nice, but note that the item is literally advertised as a “crochet olive-green loin cloth” (could just be a quirk of the seller)
