Ranking use cases and finding focal use cases gives you a good idea about where to go next in your design. It helps in reducing the risk by concentrating on use cases that are most likely to produce a design that will be of most use. The non focal use cases will either not impact the design much when implemented, or will not impact the usefulness of the system if not implemented.
However: Prioritising use cases can give a false sense of security: the system is described by all the use cases, not just the focal ones. Making some use cases a higher priority implies making others lower priority, and risking alienating any stakeholders who champion those use cases.