Writing unit tests makes you your own first customer. By covering all use cases in tests, you probe the user experience of your API before anyone else does. This is distinct from testability as a design heuristic. That’s about internal structure, while this is about the external interface. If your test setup is awkward, your API is awkward: combinatorial constructors, nonsensical method calls, ambiguous entry points all surface as pain during test writing.
API usability and testability form a virtuous cycle: simplifying the API constrains code paths, which makes it more testable, which reveals further simplifications. The tests become a feedback loop that polishes the interface from the consumer’s perspective.