The API is often the most important part users never see

When an app, website, database and external services need to work together, the API is the backbone.
api.statusonline
seo.coreready
mobile.uioptimized
cms.themeactive
An API is not just a technical word. It is the agreement for how systems talk to each other. If the API is poorly designed, every future feature becomes more expensive and riskier.

That is why API systems need authentication, permissions, data structure, error handling, documentation and testing from the start. A good API may not be visible on the homepage, but you feel it every time the system works reliably.