We have different types of applications :
1. Writer - to load data into mongodb from different data sources
2. Reader - to read data and display to end user.

Normally, there is strict SLA for reader , but no SLA (or less restricted) for writers. We want to make sure that writer will not impact reader in case when for some reason a lot of data arrived from external sources. So, we would like to slow down writers for the sake of readers.

Writers can saturate CPUs and IO, that's why we want an option to leave some room for readers. Normally it's hard to limit on application level because applications can be deployed on many different host and may not communicate to each other.