Digital platforms often grow to support several clients: public website, administration area, mobile app, internal automation or partner integrations. A central API prevents business logic from being reimplemented in every interface.
Clear responsibilities
The backend validates data, enforces permissions and implements business rules. Frontends focus on presentation and interaction.
Multiple clients
When web and mobile use the same endpoints, behavior remains more consistent. New integrations can build on existing capabilities.
Security
Authentication, rate limits, validation and audit logs can be controlled centrally. This reduces the risk that a new client bypasses security rules.
Not an end in itself
API-first does not mean splitting every small website feature into microservices. A well-structured modular application with a clear API is often the better balance between flexibility and operational complexity.