It all comes down to developer productivity. In the era of rich clients and jamstack, it's much easier for developers to work with tools from their frontend than from their backend.
Consider you offer a traditional backend API that requires a secret key for authentication. A frontend developer cannot access your API directly, so they must coordinate with a backend developer.
We noticed that more often than not, the backend developer is just plumbing. When a request comes from the frontend, they make sure the current user is authorized, but otherwise just pass the request along and return the raw response.
Frontend-first is a strategy for safely exposing your API to the frontend. It results in less work for backend developers, fewer blockers for frontend developers, and happier overall customers for your developer tool.
Frontend-first is made possible by changing the traditional API authentication strategy. Instead of using a secret key with global permissions, frontend APIs authenticate with the current user's session and scope permissions to that user.
If you're building a developer tool that stores User IDs (or Person IDs, or Customer IDs, etc), you should probably build frontend-first!
We built this website to help new and existing developer tools leverage a frontend-first strategy. The techniques documented here were learned through lots of trial and error, and we hope others will learn from them (and iterate on them!) to build better developer tools.