You are already familiar with private keys on Steem by now. But here is short analogy to remember what key stands for, what and how you should secure yourself.
Posting private key
This key is for your home, where anyone who has access to your key can enter into your home, use common goods for you because you have a level of trust to them. In the case of Steem, Voting, Commenting, Posting, Reputation
Private key is on
This key is for the safe / safe where you save your money, not many people know the password but only you. Who has access to this key can control whatever is in it safe, plus your identity. In the case of Steem, balance and fund control, account details
Owner's private key
This key is your home key and safe / safe, while giving you access or permission to own the house completely. In the case of Steem, Account ownership
Personal key memo
This key is your signature where you signed documents or messages / important letters. In the case of Steem, Encrypt messages, personal communication.
You can share any of the above permissions with other accounts in Steem.
Just like, you can rent a room in your home or have more than one passcode to access a safe / vault or joint ownership of the property / house or appoint a lawyer.
Conclusion
Always know how your private key is used and who can access it. Use with caution!