Knot density is a measure, not a verdict
Knots per square inch tells you how fine the weaving is and how much labour went into it. A fine Tabriz at four hundred knots is a different object from a village rug at fifty. But density alone does not decide value: a densely knotted rug in poor wool with harsh chemical dyes is worth less than a coarse rug in superb hand-spun wool with natural dyes. Density explains detail capability, not quality.