It is very handy to have a possibility to duplicate posts in WordPress when you work with a lot of similar posts. Especially if the posts have the same custom fields values.
It means that you do not have to re-enter custom fields, post tags and categories each time.
No matter how great Gutenberg becomes, no matter how many cool plugins appear that allow you to manage custom fields, it is always useful to know how to create a metabox from scratch. At least look at WooCommerce, the Products post type works without Gutenberg right now. And probably for your specific project you would need a post type without visual editing. That’s where meta boxes come into play.