Which should be used very sparingly, unlike the actual MongoDB, which shouldn't be used at all.
The worst imaginable outcome for a database.
But it was replaced with a much better engine (WiredTiger) with write-ahead journaling and checkpoints around the year 2016.
"Jepsen Disputes MongoDB’s Data Consistency Claims" https://www.infoq.com/news/2020/05/Jepsen-MongoDB-4-2-6/
"MongoDB 4.2.6" http://jepsen.io/analyses/mongodb-4.2.6
Joe Drumgoole Director of Developer Relations, MongoDB
I use it for application plugin configuration values which allows plugins to do whatever they want but also provides some structure just in case sweeping changes need to be made.
Basically where schemas aren't important or will be defined by someone else.