Setting a bot as Primary is important for avoiding fallback situations on the start of the conversations for all channels.
Your first bot is going to be set as Primary to start your bot's AI training. The Primary bot will be indicated with blue stripe as below:
Your first primary bot is trained with greeting queries. Open on your primary bot and click the Edit button on the Start of Bot Flow. You will see that some User Says are already entered.
Please note that after you set another bot as primary, these User Says won't be moved into new primary bot. If you want to move these User Says, you need to delete them from the old primary bot and add them to new primary bot!
You can use rule-based bot scenarios on your website and your visitors can proceed with clicking on buttons, selecting cards etc. However, on the other channels such as WhatsApp, Facebook Messenger, Instagram, your visitors can actually send text messages directly to start a conversation. If you didn't train your AI accordingly, your bot may continue with the fallback bot since it didn't understand the visitors queries.
Setting as Primary can save you from these situations since it's already trained with greeting queries.