In my experience, it's multi-faceted. First, is the ease of integration. Native plants are, well, naturally better suited for your area. If you select a flower that grows natively in your area, it will be okay as long as you match the sun requirements. Second, they take less maintenance. Less water and little- to no-fertilizing, because they're already well suited to the climate and soil. Third, is that they are almost guaranteed not to be invasive. And finally, you don't have to worry about introducing anything toxic to the native fauna inadvertently. If it's toxic, they already do not eat it.

For me, it was that they are super easy to take care of, and thrive under conditions that would murder "standard" sorts of landscaping plants. I'm forgetful, and a watering schedule is just not an option for my poor, poor brain.