Maternity dresses are designed to help celebrate your new body! We know that while pregnant, our clothes just don't fit the same, even when they are roomy and stretchy and we really want to make it work during pregnancy.
Maternity dresses often have rouching in the midsection to allow the fabric to gather on the sides in the front to flatter your bump. They're also cut longer in the front so the hemline is even when worn. You'll notice with your non-maternity clothes they often "ride up" in the front because they're just not designed to fit a pregnant person's body.