private and public are p2p answer to security, so messages don't get intercepted pretty much how people sent letter with stamps and seals, now think that only the corresponding recipient can remove the seal with his private key, because you sealed with his public key, then you know only he read the message or whoever is the holder of the other key.
It's cryptography and how blockchains work, that's how you get a trustless system, because users are not trusting the system to do anything else than what it is intended, ie. relay messages.
Cheers :)