Milkweed is the host plant for Monarchs. The butterflies lay their eggs on the the plant and the caterpillars eat the leaves. Seems like most people know that.
But did you know that the scientific name of this plant is Asclepias curassavica? And it was named after the Greek god of medicine, Asclepius. This plant has been used for "what ails you" for thousands of years. Intestinal parasites? Got it. Warts? Got it. Syphilis? Got it. And the list goes on.
So, since it was named after a god, I consider it a sacred plant.