Why do we use #include in C?

is a Processor Directive.

In the C Programming Language, the #include directive tells the preprocessor to insert the contents of another file into the source code at the point where the #include directive is found. Include directives are typically used to include the C header files for C functions that are held outsite of the current source file.

The syntax for the #include directive in the C language is:

#include <header_file>