When a customer who may have logged in and started shopping on one device (e.g. their mobile), logs into your store on another device later (e.g. their laptop), they'll find their cart waiting for them, making it easier and more likely for them to complete their order.
If you change the theme then you just need to enable App storefront feature setting then the features will start to work on your new theme. If need any support, please email us at firstname.lastname@example.org