Hi, I worked on improving the search rankings for a popular package manager. Imagine your search algorithm is already excellent, you have countless of documents, and customers do countless of unique queries. Now say that you want to improve your search rankings further. How do you do that? What if your improvement helps some queries but hurts others? Things like tf-idf or Elasticsearch won't help here.

That's where NDCG comes in! Basically it gives a score for your search rankings that you can use to compare different search algorithms. The higher the score, the closer your algorithm was to producing the expected search results. This is super useful as you can try lots of experiments and get a good sense of whether the experiment is promising or not.