For some reason the RFC focuses on idempotency, but then says it's explicitly intended for enabling caching semantics. Caching a query that mutates visible state doesn't really make sense, and like you point out if you just want idempotent modifications PUT already has the relevant semantics. I guess we haven't learned our lesson from making the original HTTP semantics super squishy.

> For some reason the RFC focuses on idempotency,

It focuses on a bit more on safety, which is why every mention of it the proposed method having the "idempotent" property is immediately preceded (in most cases in the same sentence) by description of it having the "safe" property.