A soft sash seems like the less advanced way to hold up a garment: no hardware, no holes, nothing to click. Yet it does several things a buckle cannot. The pull is spread across the whole circumference of the waist instead of being delivered at one point; the fit is continuous rather than stepped by hole spacing; and when the load becomes sudden, the knot gives way before the cloth does. Those are three different advantages, and they all come from the same decision to let the belt be fabric.
Consider where the force goes. A buckled belt is a tension member with a single weak link: the hole that takes the pin. All the load passes through a few square millimetres of material, which is why an old belt shows a shiny worn crescent at exactly one hole and eventually splits there. A sash presses over a broad band. The same holding force, distributed over a strip eight or ten centimetres wide, produces a much lower pressure on the garment underneath, and it leaves no local crease to remember.
Continuous adjustment
A waist is not a fixed measurement. It changes after a meal, between seasons when a layer is added or removed, through illness, through growth, and over a lifetime. A sash answers all of these with the same object, because a knot can be tied at any point along its length. A buckle answers in steps, and the step is usually too large; the wearer ends up choosing between slightly too tight and slightly too loose, with no position in between.
The knot is also a deliberately weak element, and that is useful. If a garment catches on something, a tied sash can slip or come undone, dissipating the jerk. A buckle will not release, so the energy goes into the fabric at the hole or into the seam that holds it. A fastening that fails cheaply and visibly is often safer than one that holds at any cost — the same reasoning that puts a replaceable cuff on the edge that wears out first.
Width and surface finish are chosen together. A narrow cord carrying the same tension cuts into the layers beneath it; a wide band does not. And a glossy, smooth weave makes a beautiful sash that slowly unties itself, because the friction holding the knot is small. Everyday sashes tend to be woven with a slight tooth or made from two layers, so the knot stays where it was put. The unglamorous finish is doing mechanical work.
Why it is wrapped twice
The usual method — one turn to set the circumference, a second to bring the ends round to the front side before tying — is not just neatness. A knot sitting at the centre of the abdomen is pressed against the body by a chair, a table edge or a carried load, and it becomes a hard lump exactly where the body bends. Moving the knot slightly to one side takes it out of that line. The second wrap also lets the first turn be pulled snug and then locked by the friction of the turn over it, so the tension is set once and does not creep.
This is close to the reasoning behind wrapping a baby rather than dressing it. In both cases a flat length of cloth is made to fit a body that changes shape, without cutting, without sizing, and without hardware against the skin. A frog fastener is the related solution where two edges must stay aligned: it uses the stiffness of a cord and a loop instead of a rigid pin, so the closure can flex with the layers it joins.
Ceremonial dress complicates the picture in an instructive way. A stiff, decorated belt with plaques or a clasp does appear over the soft sash, but it is not doing the main work. It carries rank, hangs ornaments, and holds the outer silhouette; the cloth underneath still takes the tension. The two belts have divided the job — one holds, one signifies — and confusing them leads to reading the stiff one as a better fastener when it is really a different kind of object.
Modern elastic waists solve the adjustment problem a third way, by handing it to the material. That is convenient until the elastic ages: the fit was never a decision the wearer could renew, so when the rubber loses its recovery the garment has no fallback. A sash has no part that can perish in that way. It is a length of cloth, and its only mechanism is the knot, which can be retied as many times as the garment is worn.
So the absence of hardware is not a missing invention. It is a choice about where force should go, how fine the adjustment should be, and what should give way first. Once those three questions are answered, a soft band tied at the side is not a simpler version of a buckle — it is the correct part for a garment cut from rectangles that were never fitted to one body in the first place.