It's not so much about caching - it's more about knowing that you're safely able to repeat a request which timed out or failed.

A way to distinguish form submissions (like a credit card purchase form) which must not be repeated after submission, from things like "upload a new version of this file" which you can safely allow to be retried if they time out or fail.

This is potentially useful for a bunch of different HTTP clients and client libraries, and potentially for some proxies too.

Edit: actually PUT and DELETE are useful for caches - although not because of their idempotency, but just because they communicate the fact that the resource being PUT to or DELETEd may change as a result, hence the cache key for that URL should be purged. IIRC some caching proxies do do this.

Hehe, a proxy cashing PUTs can spice up your REST experience. How exactly will the proxy know someone else haven't updated a resource between 2 requests and it's safe not to pass the 2nd request to server?