[kentonv]

A few cases off the top of my head:

- When you need to work with older browsers or non-browser clients (e.g. API clients!) that don't support service workers.

- When it would be a security problem if the user can bypass or interfere with the worker (e.g. you can implement authentication in a CF Worker).

- When you specifically want to optimize your use of the shared HTTP cache at the Cloudflare PoP.

- When startup time of a service worker would be problematic.

- When CORS would prevent you from making the requests you need from the browser side. (CORS doesn't apply to CF Workers, since the things CORS needs to protect against are inherent to the client-side environment.)