1. It's easy to maintain/update the code because it is pushed once to Cloudflare and you don't have to worry about browser caching effects on JavaScript delivered to the browser.
2. The performance of the code will be much higher than in the browser because of the server resources available and also because and subrequests will happen across Cloudflare's fast/reliable links and not whatever the end user is connected to.
3. The end user has control over what JavaScript is executed and might use a tool like Disconnect to block external scripts preventing the code from running at all.
4. Security: you can include things like API keys.
5. Script starts executing earlier.
6. Conserves bandwidth/battery life of mobile users.
- 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.)
It's lower latency once you've delivered the javascript.
There's a lot of interesting stuff you can do with edge applications. Image optimization/resizing, content rewrites, pre-rendering, API gateway, etc, etc, etc. These are all things you want to do once for many visitors.
There's a lot you _can't_ do in a browser because browsers are untrusted. Edge applications can run with a different level of trust.
