Why do leafcutter ants carry cut leaves into their underground nests?

Answer: They use the leaves to grow fungus that feeds the colony

They cut the leaves to sharpen worn jaw edges — Cutting leaves wears jaw edges rather than sharpening them like a whetstone. The benefit comes from using the harvested material to cultivate food. Sharp jaws are tools for that harvest, not its goal.

They use the leaves to grow fungus that feeds the colony ✓ — Leaf fragments provide growing material for the fungus that feeds the ant colony. Workers process the plant material and tend the fungus in underground chambers. The leaves are a crop substrate, not simply ready-to-eat leaf meals.

They use the leaves as waterproof roofing over the nest — The harvested leaves are not chiefly roofing material. Workers take them to fungus gardens inside the nest, where the fungus can grow on the prepared plant matter.