Because you want to show your sincere apologize when you have done something wrong or make someone feel unhappy. You will say," I am really so sorry for what I have done to make you angry. Hope can help. Sorry means 对不起 in chinese.

谢谢你 友達 John:)!!

我 很 抱歉 is for more serious fault than 对不起

just like "i am sorry " with "i am so sorry" 对不起 is simple apologize. 我很报歉 is when you hurt someone.and you hope people can forgive you.or something you can't do .nothing eles you can help.

They can be used interchangingly on most occasions. But 对不起 is more formal and serious. When it comes to something big, you can say 对不起。