C

why do we need return 0 in c?

The return value is the exit code of your program, the shell (or any other 
application that ran it) can read and use it.
The 0 exit code is a widely accepted convention for 'OK the program execution 
was successfull'. And for non-zero numbers, it conventionally means the 
program didn't execute successfully or there is an error.

why there is return 0 used in c

The return value of main() becomes the exit status of the process.
Traditionally, an exit status of zero usually means “OK,” 
while any non-zero value indicates some kind of error. 
This is analogous with how many system calls likewise return zero 
or an error code.