Basically when speed and horizontal scalability are very important, and consistency/durability are less important. It’s also pretty good for unstructured or irregularly structured data that’s hard to write a schema for.

Web scraping or data ingestion from apis might be a reasonable use case. Or maybe consumer apps/games where occasional data loss or inconsistency isn’t a big deal.

It can also be used effectively as a kind of durable cache (with a nice query language) in place of redis/memcached if you give it plenty of ram. While its guarantees or lack thereof aren’t great for a database, they’re pretty good for a cache.

I worked for a big news website- we used mongodb for pixel tracking data.

I went on to work at mongodb