1. Lets you keep up with a faster lane during swim practice
2. Easy way to sneak some bonus ab work into a workout without doing anything else
3. It's good to feel comfortable when you're uspide down in the water and disoriented while being forced to hold your breath. Because you will get disoriented in rough water situations when the surf gives you a tug.
4. Easier on the back and shoulders than open turns are.
5. Because even if you think you're quick on your open turns, you're still getting rest on them you wouldn't get doing flip turns. An extra second per 25 or 50 adds up to a huge amount of 'stealth rest' when you're doing 250s or 300s. Turns force you into more continuous swimming.