API work tends to arrive from one of four directions, and each one changes what gets built first:
- There's an app, but no backend. A mobile or web client needs somewhere to authenticate, store data, and run business rules. The API is the product at that point — the screens are the thin part.
- Customers want to integrate with you. A public API is frequently what closes a larger deal for a SaaS platform: the buyer's team wants your data in their warehouse and their tools, and a CSV export is not an answer.
- Your own systems don't talk. The CRM, the finance system, and the operational tool each hold part of the truth, and somebody is keeping them in sync by hand every morning.
- The existing API has become a liability. Undocumented, unversioned, and breaking a client every time it changes — so nobody dares touch it, and it slowly blocks the roadmap.
In the first case we usually build the API alongside the product it serves, as part of a wider web development or mobile build. In the last case the job starts with reading what's there: mapping the current endpoints and their real consumers before proposing a version that can be migrated to rather than swapped in overnight.