The CSDM White Paper attached should answer your questions. For more info see CSDM product docs site and follow CSDM Community for white paper updates.

The short answer is that these 'management' suites cater to different personas in the organisation. For example, Application Portfolio Management is intended for Enterprise Architects. The Strategic Portfolio Management is intended for Portfolio and Project Managers etc. Service Portfolio Management is for Portfolio Owners and Service Owners. These owners can structure and design services for managing and tracking - i.e. SLAs, catalogues, surveys etc. 

ServiceNow is able to provide a source of truth to all by interrogating the same data. It also has the added bonus of being able to inform different personas about things happening in their world and outside their world that they would need to know.