Four hundred tests. Each logs in through the form. Each login takes three seconds. That is twenty minutes of every run spent typing the same password, and it is usually the single largest line item in a suite's runtime.
You met cy.session in the intermediate track. This lecture is about the parts that matter when you have hundreds of tests and several user types, because that is where the caching rules stop being a detail.
The mental model. cy.session runs your login once, then saves the resulting cookies, localStorage and sessionStorage under a key you choose. Next time that key appears, it restores the saved state instead of running the login again.