The reason for using end-to-end encryption, instead of encryption which is not end-to-end, it to protect against the service provider. That is, if Whatsapp's encryption is really end-to-end, you don't have to fear that the Facebook servers might have been invaded by evil hackers intending to leak your most private communications to the whole world; the evil hackers would have to invade your personal device directly (and they can't invade everyone's personal devices, since that risks exposing their evil misdeeds to security researchers).
