Tags are customizable labels you can apply to your contacts to help organize your audience in a way that makes the most sense to you. You can use tags as the basis for building audience segments for sending to targeted audiences or even create an automation that triggers when a specific tag is added to a contact.
An example: You want to add the contacts generated by a certain flow to an audience in your email service (for example Mailchimp or something similar) in order to be able to send certain emails only to these people. Use tags to identify them and automatically assign them to the right segment during synchronization with the help of Zapier.