If you look at the first example in the cdkPortal docs, you will notice one big advantage/commodity:
cdkPortalOutlet unifies the functionality of ngTemplateOutlet and ngComponentOutlet by allowing devs. to embed both templates and components into the view dynamically.
Furthermore, ComponentPortal allows to dynamically pass Injector instances, which can become handy in cases where you need finer control over the dependency resolution of the to-be-embedded component.