There are several different use cases. The most basic use case would be just to be able to share data from a database or a data repository. That's the most basic use case.
Microsoft Power BI is a visualization tool, is a BI tool. There are more than 1,000 use cases that you could use. There are countless use cases for which a BI tool or visualization tool could be used for.
The simplest use case is where a colleague in an organization who does not have any coding skills or does not have any technology background wants to be able to look at some data from a database or a repository of data. He or she could use Power PI just to be able to connect to that system and just take a look at or peek into that database. It is as simple as that.