Key confirmation and other benefits of double ratchet
This is convenient because each new key is used one-time and depends on the root key and message, so you do not need to constantly announce the next new key and wait for confirmation.  If Bob reads the encrypted message, he knows the encryption key and can decrypt the next message.  Since by exchanging messages that need to be decrypted, you get the opportunity to know how to decrypt the next message.  This is what is called “future” encryption in the article.  Thus, we have only one root key and constantly changing ephemeral keys, which are obtained from the messages themselves.  This allows you to simply communicate while being sure that the protection is very reliable.  I recommend reading the source code for this article from 2013.  Maybe this can clarify the principle.  A potential attacker, even if he intercepts the original message and all subsequent ones, will not be able to push off from something to start the attack, because he will have messages that are always encrypted with different keys and depend on encrypted messages that need to be decrypted.