The Edge layer
Middleware sits between the user and your React application. It runs on the Edge, meaning it has cold starts of <10ms. This is the perfect place to make routing decisions based on cookies or headers.
Advanced Auth Pattern
Don't just check for a token. Refresh it. If an access token is expired, middleware can perform the refresh rotation with the auth provider and set the new cookie on the response, all transparently to the client. This eliminates the 'flicker' of client-side auth checks.
A/B Testing with Rewrite
Instead of conditional logic in your components, use middleware to rewrite the URL. Provide a `X-Experiment-Bucket` cookie, and rewrite `/pricing` to `/pricing-b` for 50% of traffic. The user sees the same URL, but gets a completely different page file.
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