Why can pale mold on dry-aged beef be useful during aging instead of just a sign of spoilage?

Answer: The mold secretes enzymes that break down meat proteins and connective tissue.

The mold forms a lipid-rich barrier that prevents water loss from the meat. — Not quite. Dry aging does dry the surface and form a crust, and that crust can slow some microbial growth. But it is not a lipid-rich seal that prevents water loss; dry aging depends on water evaporating from the meat. The useful mold effect is enzymatic, not a water barrier.

The mold metabolizes sour-tasting acids into milder flavor compounds. — No. Some microbes do change acidity in fermented foods, so the idea is not random. In dry-aged beef, though, the useful mold action is mainly enzymatic breakdown of proteins and connective tissue, not conversion of sour acids into milder compounds. If mold, humidity, or sanitation go wrong, the same surface can become spoilage.

The mold secretes enzymes that break down meat proteins and connective tissue. ✓ — Right. In controlled dry aging, some surface molds and yeasts secrete enzymes that break down proteins and connective tissue near the surface. That enzymatic action can make the beef more tender and help develop flavor. Temperature, humidity, and hygiene keep the mold in the useful range; without those controls it can turn into spoilage.