Well, the public keys are published on the web, whereas the private keys are only locally stored on your computer or your smartphone. The private keys are never published online, but known only to the clients, or owners of those keys.
What if you lose your smartphone? You also compromise the security of your private key.
One credible way to strengthen the security of private keys is by encrypting them with fingerprints.
Fingerprints are unique to a person, so unique that even twins have different ones. Moreover, you always carry your fingerprints with you. No one can steal them. Impressions of fingerprints could be stolen, but research is going on to improve fingerprint scanning and feature extraction algorithms that will distinguish between live fingerprints and their impressions.
So, essentially the unique features extracted from a scanned fingerprint are used to encrypt/decrypt the private key, making it more secure.