. That's why the Monarch butterflies which is the best known member of the milkweed butterflies is flocking to sip the nectar of the plant, hundreds of them together, a reward for the beholder!
Monarch butterflies and milkweed butterfly plants are closely attached to each other. Most adult monarch butterflies (pictured above) use the nectar-rich milkweed as food source. It's the only plant material the Monarch caterpillars can eat. Without them, they will starve, get sick and die. (Thanks to Wikipedia for the information).