Why here use a closures to set value for shader's uniform?

Question

I'm looking at GPUImage3, My question is why closures are used here instead of direct assignments

public class LuminanceThreshold: BasicOperation {
    public var threshold:Float = 0.5 { didSet { uniformSettings["threshold"] = threshold } }
    
    public init() {
        super.init(fragmentFunctionName: "thresholdFragment", numberOfInputs:1)
        
        ({threshold = 0.5})() //this line
    }
}

I want to know why and how it works.