A lot of people will argue that state helps protect against drift, but the real reason I find that you have to have state is to store values that won't be returned a second time and still construct and connect the graph of resources in the IaC templates. For example, if you declare the need for an RDS database and connect its output credentials into another application, you'll need state in order for the applies to work a second time because you'll never be able to retrieve the values from the target provider again.