Outbound custom bots allow you to engage via a type of message (it is not really the same as a post or chat) which displays the first bot dialogue vs waiting for a user to engage - could be a message, message with video, bot dialogue options etc.

Messenger allows for you to send posts/chats/etc but they are a one off thing (unless in a series or campaign) and you dont get the option to go through bot dialogues etc.