One FAQ is "Why do we even need symmetric cryptography? Why don't we just do everything with public key cryptography now?" It's because public key cryptography

One FAQ is "Why do we even need symmetric cryptography? Why don't we just do everything with public key cryptography now?" It's because public key cryptography is extremely CPU resource-intensive, while symmetric cryptography is not. So we typically use public key crypto only to exchange symmetric keys, and that way we get the best of both worlds without chewing up too much CPU time.