Not only are they beautiful, but butterflies are essential in pollinating and their presence indicates that your garden is healthy. Sadly, this year it was announced that Monarch butterflies are on the endangered species list (by the International Union for the Conservation of Nature). The reasons are from habitat loss, overuse of pesticides, and changes in weather. You have the power to keep them around by planting their favorite foods and considering less harmful ways to prevent pests.
Butterflies are attracted to flowers, so choose a variety of local ones. I have had lots of success with a wildflower seed mix this year and it is the most budget-friendly and safer option for your soil. They need a safe place to drink water too. I've seen them drinking from hummingbird feeders, from citrus slices, and shallow dishes that you can make yourself!