Why int 21 h is placed in ms dos system?

INT 21h is an interrupt in MS-DOS that provides a wide range of system services, including file handling, device management, and memory management. It acts as an interface between user programs and the operating system, allowing applications to perform tasks such as reading from and writing to files, managing input/output, and accessing system information. By providing these services, INT 21h simplifies programming and enhances the capabilities of DOS applications, making it easier for developers to interact with the underlying hardware and system resources.