It encourages custom verbs that are universal independent of the media type. That way, the operation of the verb (idempotency, caching, etc) can be fully specified and applies across the board.

If I do a GET, I know that I can do it again without affecting the resource. I'll GET the current state of the resource identified by the URL.

The RFCs define those verbs and their operation. I can rely on that spec for the way they operate.