What is the use of /usr/include directory?

That folder includes the header files for C compilers. Such as "stdio.h", "stdlib.h" etc.

When you type header information in the C source file such #include <stdio.h> the compiler will look for the file in /usr/include directory by default.

big_endian.h and little_endina.h files are included, because though your computer may be of only one kind, you can cross-develop application for both architecture. So, you need both header file.

• What is the meaning of the default directories in the Linux filesystem hierachy?

• See this link about gcc's searching preference

• Another page explaining the /usr/include directory (The original link was removed, this is the one that's cached by archive.org)

These files are needed when you compile programs, be that a software package you need to compile manually or your own programs. They are included in the C code such that you can use the functions defined there. Don't worry about them if you don't code.