It's super useful for many reasons: - to help make thread-unsafe APIs thread-safe E.g., errno, strtok(), and so on. - to store thread-private mutable

It's super useful for many reasons: - to help make thread-unsafe APIs thread-safe E.g., errno, strtok(), and so on. - to store thread-private mutable globals (Thread-locals are more like thread-globals.) - to keep a reference/copy of global immutable data that might get replaced in the middle of a thread's handling of a request - to keep contextual data that you can't retrofit an API to pass explicitly (see also first item)