what is the use case/purpose of this?

The idea is to introduce a

ThreadLocal

variable to be shared between coroutines. This variable can be read from a running coroutine with

coroutineContext

.

ok so do I understand correctly it is a way to share data between threads when using coroutines?

Coroutines offer an abstraction over using threads. They focus more on communicating between coroutines than sharing state between them. The main primitive to communicate between coroutines are `Channel`s : https://kotlinlang.org/docs/reference/coroutines/shared-mutable-state-and-concurrency.html