I'm recently working with microservices in Asp.Net Core. With microservices, there tends to be a lot of network connectivity. One service potentially calls many other services to build a final response. It's difficult to see where a request started, all the subsequent requests that happened back to the final result. Concurrency keys are a way to track a single request through all its dependancies.
The idea is when a request is made a concurrency key is generated.