I'm in the process of building my own search engine (as a learning exercise, but also because it's related to my day job). I've learned that it's one thing to write a full-text search engine, like this one, and it's quite another to do field-specific searches with faceting support and so on, like Algolia and Lucene-based search engines do.
That said, this is clean and simple. I like it. I can definitely learn from this.
Tracking (document, field) values can be used for query by range or by geolocation primitives (that's what Lucene does, where it will index that data into a special tree-like structure, and for each query, it will build a custom 'iterator' and use it along with other iterators to match documents), and for static ranking of matched documents.
BTW, Lucene and Algolia are vastly different in terms of the underlying architecture.
As Mark mentions on his summary page, the best place for that kind of information is our CTO's "Inside the Engine" series (8 parts).
https://blog.algolia.com/inside-the-algolia-engine-part-1-in...