These two blocks have some magic behind them.
The «Welcome block» is always shown to all new users after they tap «get started» or send their very first message. You can keep the link from the welcome message to the main menu, or you can delete it if you don’t want to show the main menu after the welcome block.
The «Default answer» is like a «homepage» for your bot users. This block is shown when a user sends any message (except for messages that are being recognized by the “AI Setup” tab). You can answer to some user messages with other blocks. Go to “AI Setup” tab to choose what block to show based on your user’s input.
You can make your bot transparent – by adding «Live chat» plugin to «Default answer» block. Now if your users types anything not recognised by AI, he will get into live chat session with you as Facebook Page admin.