Crafting a high-quality Email can be difficult as it is not possible to use custom variables in transactional email in Magento 2 by default. The store owner may want to add customer-specific details, additional order details, etc. for delivering more information or for a personalized approach. 
Visit https://meetanshi.com/blog/use-custom-variables-in-transactional-email-in-magento-2/ for the programmatic method to use custom variables in transaction email in Magento 2.