If you see a lot of butterflies, then use of pesticides probably is low and variation of plant growth high. If you see few butterflies, then there is probably a lot of pesticides, pollution and few plants. Butterflies are also an important part of the food chain and the larger food web of any given area. They are also important pollinators. For millions of years, plants have evolved a delicate relationship with insects.
Flowers produce nectar to attract insects.
This movement of pollen around the flower increases the likelihood that seeds will be produced in the female part of the flower. The relationship can be so specific that some plants in the world can only be pollinated by one species of insect. Host plants are food plants for caterpillars. Again, there are some butterflies that will only lay eggs on specific plants for their caterpillars.