Hello ,
Specifically SRM UI add-on is simply a Add-on solution to have Shopping cart in new UI Experience for desktop. These were conceptualized as part of SRM Suite of applications.
These are apps in alignment SAP Fiori suite( A newer UX ) and targeted also for mobile and desktop . These follow the SAP suite of Fiori .
Also some of backend services are from former app is reused so the former add-on is a pre-requisite for the Fiori Apps.
Thanks and Regards,
Veera