Why we use curly brackets {....} In many programming languages like C/C++ ...

Question

Between main program and sub program

Answer 1

This is the language standard, some languages use them some don't. C uses them to enclose a group of statements. Take a look :

https://www.computerhope.com/jargon/c/curlybra.htm

In certain cases, they can be omitted :
https://stackoverflow.com/questions/14901919/when-can-i-omit-curly-braces-in-c/47531901