It's the difference between type checking and business logic validation. E.g., Type check that the user's name is a String and then let the database/business-l

It's the difference between type checking and business logic validation. E.g., Type check that the user's name is a String and then let the database/business-logic yell at you when it's not unique. In this case, checking that an email address input is "shaped" like 'not-empty@not-empty' is the "type check", and then actually sending an email is the only way for your business logic to know if it's actually valid.