Convenience of only needing to take one item with me that can store my most used credit cards + id, and a bit of cash which I rarely ever need to use anymore. I use the BookBook case for my iphone 6+
For me I usually carry a small purse as I need to take my car key fob with me. I also very rarely carry my phone on m pocket. If I'm exercising, I use a flipbelt. So for my use, I like a super thin case for grip and scratch protection.
The wallet case is enticing though.