What functionality is missing? What is unsatisfying with the current situation?

When the same information needs to appear in multiple locations, then that text needs to be maintained individually in every article where it appears, except in specific cases where World Anvil supports re-using content across multiple articles (like content trees, statblocks, and tables).   This means that larger articles that include sections from other articles (like an article about a city that describes its key landmarks and people of interest) are difficult to maintain. Information in one article tends to become incorrect or obsolete if updates are not made to each article.   The broader article can avoid describing any of its constituent topics and simply link to the relevant articles, but this results in the reader opening many tabs or constantly going back and forth between articles, which is a poor presentation of information that is meant to be presented cohsesively for a single reading.  

How does this feature request address the current situation?

The ability to use MediaWiki-like transclusion would allow a single authoritative article to be maintained, and then content from that article transcluded (embedded) into other articles where it is relevant.  

What are other uses for this feature request?

MediaWiki projects also use transclusion to include features such as standardized footer sections for information that is identical for all articles on a similar topic.