When you build a chatbot it is important to reply to this question: Do people really need to be able to send messages or can you drive the conversations with your bot just by using buttons?
If users need to write a message, talk to you or if you wish to reply to them, you must activate this option. This is the classic way messenger works already without a chatbot.
Now If you are looking to generate leads, or want to display content, display FAQ, just send content every week and don't want to pollute the user experience (or get lots of message your bot won't understand) you can switch this OFF.
You can of course switch this ON and OFF very easily.
ON: people can send you a message
OFF: people can't send you a message
If you do switch this ON, make sure you have an unanswered recipe with buttons that drives the conversation.
You can find this option in the Bot configuration page > Persistent Menu