MongoMK (realized through MongoDB) was introduced to address the need of scaling author nodes horizontally. General notion is that a typical author node supports 30 concurrent users. What if there is more than 30 concurrent users when a single machine’s capacity (processing power and the memory) is not sufficient. There comes the typical use of MongoMK based clusters where multiple AEM instances can share the common node store addressing the horizontal scale of active-active configuration. Other use of MongoMK would in case of asset deployments and community deployments.
Source – adobe.com
Another useful resource – Microkernel – Which one to use