Ask HN: If you use MongoDB, is there any reason to use Kafka?

I ask in all seriousness. I have the impression that when Kafka was first adapted by organizations, many organizations were looking for a place in their data flows where they could store documents instead of fully normalized data, and Kafka emerged as a respectable answer.

Emphasis: I am not asking if MongoDB is better than Kafka. I am asking, if your organization is already using MongoDB, is there any reason to also use Kafka? Is it easier to simply press MongoDB into service in a similar manner (similar to Kafka)? One can have log-like append-only collections in MongoDB.

Back in 2013, Jay Kreps explained his reasons for creating Kafka:

https://engineering.linkedin.com/distributed-systems/log-what-every-software-engineer-should-know-about-real-time-datas-unifying

I first read that essay in 2013. I just read it again, and it occurs to me that I can use MongoDB for everything he describes.

So under what conditions would I still want to bring in Kafka?