It's like a "dumbphone" for people who still want to be able to access some of the features available through the internet, like Facebook, Whatsapp, and maybe even some light Google Maps (not that it does live guided driving directions). But it's also designed to be easy to use, with big buttons, and is designed to give better battery life than a smartphone. Like an old phone, combined with a bit of the modern thrown in.
It's perhaps for people who want a mobile phone to be a phone, more than a mini-computer in your pocket. It's also going to be ideal for people who prefer a camera to be a camera, and a phone to be a phone. So the Nokia 2720 Flip has a 2mp camera, but it's not something you're going to want to use for real photographs. Maybe you'll take the odd photo in sunny conditions. But if you want a phone to remind you that you have a better camera available, then this would be ideal for that.
You can even replace the battery if you want a spare battery, you can even add in a MicroSD card, or use a second SIM in the phone.
There's something about a flip phone's design, that makes it a modern day antidote to the "always-on" smartphone, and the always on connection to the internet. You can close the phone, and hide it away from view, as something that doesn't disturb you every time something happens on the internet.
I guess the bigger problems come when you do want to use the internet side of the phone, and messaging without a keyboard (or a touchscreen) becomes quite a sluggish process, and it can take a while to get used to typing with a numeric keypad, especially if you're used to using a smartphone. It's also not going to give you the same experience as smartphone, but as long as this is what you want, then it could be a neat option. You can setup Facebook, Email, Whatsapp, Twitter, Youtube, and add extra apps if you want.