A quick search didn't turn up one, so here's my take at it.
It can be used for a single import, if you're migrating from YNAB to beancount. But it also makes use of metadata so that it can do incremental imports. That allows you to keep using YNAB side-by-side with beancount (for instance, if you like YNAB's mobile client or budgeting approach) and only import the new transactions since the last import.