A typical product would require integrations with several existing APIs, and potentially some new ones. These would be aggregated (and normalised) into a single schema built on top of GraphQL. Then the team would build different client UIs and iterate on them.
By having a single queryable schema, it's very easy to build and rebuild interfaces as needed. Tools like Apollo and React are particularly well suited for this, as you can directly inject data into components. The team can also reason on the whole domain, rather than a collection of data sources (easier for trying out new things).
Of course, it would lead to performance issues, but why would you optimise something without validating it first with the user? Queries might be inefficient, but with just a bit of caching you can ensure acceptable user experience.