What is the main role of single-threaded programs in development and in what situations are they required?

Single-threaded programs execute tasks sequentially in one thread, which simplifies development and debugging since there are no issues with synchronization and race conditions. They are suitable for applications with simple logic where parallelism is not required, such as command-line utilities, scripts, or small services. Single-threaded programs are also often used in situations where execution determinism is important or resources are limited.