Tie back design can give you flexibility to wear it tighter and high around your waist or longer and loose, whether your super skinny or have some curves, this top flatter the figure
Open back design can give you a bit more ventilation/air circulation when you start getting hot, the slit in the back makes it breathable and breezy, perfect for a sweaty work out
This is a perfect work out shirt, the fabric is so soft, comfy, breathable, cute and of good quality, it could be worn casually or to the gym, definitely a great all-around trendy top