The detail that best supports the main idea of the section "Why this matters" is:
"Butterflies pollinate flowers, control plant populations by munching on leaves as caterpillars, and provide food for other insects, birds, amphibians, and mammals."
This detail highlights the significance of butterflies in the ecosystem, emphasizing their roles in pollination, population control, and as a food source, which illustrates the broader implications of their disappearance.