One of the benefits of summer day dresses is that they are super comfy for those insanely hot and humid days. That's because they are typically made of lighter weight fabrics and made to fit a bit more loose. However, some of them look like you just threw on a potato sack. Sure, they are comfy but you're left looking like you have no shape underneath.
The quickest way to fix that is to add a belt. It will cinch the loose fabric around your waist and give you some definition. When adding a belt to a loose fitting dress, you'll need to spend a few minutes playing with how you want the fabric to sit in/around the belt to make sure it's even.
Additionally, the belt is such a great way to add some character to your dress and look. You can easily build your accessories around your belt to suit your mood and/or occasion.