On a hand-knotted rug, the fringe at each end is simply the exposed, unwoven continuation of the warp threads that run the length of the rug — it is structural, not decorative, and the knots are literally tied to those same threads. On hand-tufted and some flatwoven rugs, where there is a separate backing or the warp doesn't naturally extend past the pile, fringe is often sewn on afterward purely for a traditional look.
Why it matters to a buyer
Knowing whether a rug's fringe is structural or decorative matters for care instructions and buyer expectations — structural fringe can be re-secured if it frays since it's tied into the rug's foundation, while sewn-on fringe is a trim detail that can simply be removed or replaced if a buyer prefers a fringeless look.