You can send numerous emails based on customer activities, order status. For example, when customer change order status from pending to fail, the extension will inform them to reorder. Or after customer abandons their checkout on your site, you can send an email to give assistance. Besides, you can send email for customer birthday, abandon the cart, last customer login, customer subscribe to the newsletter, etc. Secondly, instead of sending for individuals, by using magento 2 follow-up email you can send an email to many customers at the same time. For example, you can send a reminder email to customers who haven't visit your site for a long time with the promotion.