To understand why a using a computer system without permission causes problems, you have to understand the basic goals of information security. There are six different aspects of information that needs protection. Let's look at these one by one. Let's look at these one by one. Security experts talk about confidentiality.
Confidentiality refers to limits on who can get what kind of information.
If someone were to find that out and tell other people, that would be a breach of confidentiality. Another kind of protection for information is the preservation of control. Your parents would be frantic. They would be worried because they would no longer have control over their own secret number and over their own bank accounts. Even if they didn't do anything, you would still feel comfortable.