Over the years, PGP has evolved from a tool used largely by crypt-weenies to a widely used commercial product.
People use public key cryptography so that they can have secure, private communication.
Since the individuals were typically known to each other, trust was implicit in the personal acquaintance and users maintained their own keyring with known public keys. Furthermore, suppose that one of my friends, Alice, gives me a key from Bob. I may or may not choose to trust keys that Bob gives me. The extent of this web of trust is up to me. Certificates are the preferred way today to build a scalable network to allow two relative strangers to securely, privately communicate with confidence.