Security is your part of real life. Imagine how it would be to play in school during breaks if everyone wore a mask, had the exact same voice, and same clothes... it would not be much fun - because your friends would have no Identity.
Passing messages in class to your friends in the row behind is more fun if you can make them secret from your teacher by using a code language to encrypt what only that friend should see.
To lock your house and piggy bank, you already use keys.
When you hand in a test you sign it with your own name, to say "I wrote this" (unless you cheated and your friend helped you with answers). Your passport (and money..) is "signed" by the government of your country.
Security for 0s and 1s provides what you already use in your physical reality for the digital age based on provable mathematics.