Whenever customers take an action or reach an important milestone, it's important to be able to immediately message them, for example:

• A customer's renewal date is approaching... and you send a reminder.
• A customer leaves items in their cart... and you remind them afterward.
• A customer requests a password reset... and you instantly send a reset link.
• A customer pays for an order..... and you follow up with a confirmation message.

These messages are what we would call 'transactional', 'operational', or 'event-based' messages.