The use of associated values in enum case

Swift Programming: The Big Nerd Ranch Guide (2nd E 14. Enumerations
1

I can’t figure out the necessity of using let when checking for cases in a method (e.g. area()). I thought if it would be “Value binding” in a switch statement, however enum cases are not used within the case body, and why create a constant of enum case? Obviously the code won’t work without let, but I don’t understand why? The example ShapeDimensions :

enum ShapeDimensions {
    // point has no associated value - it is dimensionless
    case point
    
    // square's associated value is the length of one side
    case square(side: Double)
    
    // rectangle's associted value defines its width and height
    case rectangle(width: Double, height: Double)
    
    func area() -> Double {
        switch self {
        case .point:
            return 0
        case let .square(side: side):
            return side * side
        case let .rectangle(width: w, height: h):
            return w * h
        
        }
    }
}
var squareShape = ShapeDimensions.square(side: 10.0)
var rectShape = ShapeDimensions.rectangle(width: 5.0, height: 10.0)
var pointShape = ShapeDimensions.point
2

When your enum is for instance square and you invoke on it area(), inside switch case square block, how you are supposed to retrieve the associated value (not known at compile time) side? That’s where let value binding comes in to play