what is the purpose of using main() ?

Answer 1

main is entry point for your program.
system call your main function to start execution.

Answer 2

when the program is being executed , after loading all the libraries and macros, the main funtion will be called implicitly inorder to start the execution.

Answer 3

the starting point of program execution is from main. It is built like that. Hence main it is mandatory to include main.