The short answer is that the Smartcase is removable from the Wallet.
Now, for example, if you want to hand your phone to someone to take a photograph of you with your significant other, you can snap-off the phone, with protective Smartcase, from your Wallet, and hand it to an obliging photographer without relinquishing your Wallet. When finished just Snap-On the Smartcase back to the Wallet.
Or, if you are taking a photo yourself, you can either take it with the phone in the Wallet, or snap-off the case from the Wallet for improved maneuverability.
And remember when phones were used to make phone calls? Well, with the Modular Wallet you can make or take a call with the phone in the Wallet, or just snap-off the case from the Wallet for more convenience.
Another example is if you want to watch a movie or simply enjoy social media you can remove the SmartCase from the wallet, set the wallet back in your pocket/desk/nightstand and enjoy the content without the added wallet weight and size.
The modularity gives you freedom to use the wallet and SmartCase together or separately as you choose.