What are these private keys? If you run 'security find-identity -v /Library/Keychains/system.keychain' command from your terminal, you can see available/valid identities from your macbook with these private keys.   

What are these private keys?  

If i expose them, are my certs any good?  

I am writing a script which collects certificate information from macbook. Is there any way to collect these info (Issuing CA, Issued-to User Name, Expiry Date, SHA1 or SHA256)?  

Thank you.
Private keys are used for decrypting your data (message, email, picture, website, whatever you imagine). Anyone with public key can send you securely message because only you (the private key) can decrypt encrypted message. If you expose private key you will lose those privilege. Anyone on the route with private key can read what someone sent to you. But I don't have any experience with Keychain API so I can't answer your last question.