#include is used in C programs to include lines of code from external files called header files.
These header files contain functions and macro definitions that can be reused in a program, saving time and making the code more efficient.
Header files also provide data type definitions, constants and variables that are used in a program for the same purpose.
These header files also provide access to library functions, predefined variables, and other related information.
The inclusion of header files allows the programmer to write specific code that is easy to read and maintain.
Header files also enable the reuse of code across different projects, as each header file may be included in multiple source files.