What are these private keys?
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.