Why are POSIX threads used in iOS development?

POSIX threads (pthreads) in iOS can be used for:

1. Low-level multithreaded programming: They provide fine-grained control over creating, managing, and synchronizing threads directly, bypassing high-level abstractions like Grand Central Dispatch (GCD) or Operation Queues.
2. Integration with C/C++ code: They are convenient when working with existing libraries or code written in C or C++, which actively use pthreads.
3. Specialized tasks requiring direct control: In rare cases, when very specific thread behavior or optimization is needed, which cannot be achieved with GCD/Operation Queues.
4. Porting cross-platform code: If the project is developed for multiple platforms using pthreads, it facilitates porting thread-related logic to iOS.

However, in most scenarios of iOS development, it is preferable to use high-level APIs such as GCD or Operation Queues, as they are safer, easier to use, and better integrated with Apple frameworks. Using pthreads requires a deep understanding of multithreading and can lead to hard-to-debug errors, such as data races and deadlocks.