The purpose of a pre-processor in programming languages is to modify source code before it is compiled into an executable program. Pre-processors perform tasks such as file inclusion, macro substitution, and conditional compilation, which streamline the code and eliminate redundancy. For example, in C/C++, the pre-processor handles directives like #include and #define. This added layer helps enhance code readability, manage complex builds, and enable cross-platform compatibility, making it easier for developers to write and maintain code effectively.