Plugin extension closure is not callable without explicit .call()

[smac89 (Chigozirim C.)]

With regards to this question: https://stackoverflow.com/questions/40172625/passing-a-closure-in-a-plugin-extension-in-gradle.

In build.gradle

apply plugin: TestPlugin

testConfig {
    closure = { println "Modified closure!!" }
}

Run the test task.

Why does the closure not get called when simply invoked, but only gets called when explicitly called with .call()?

Similar question asked on Stackoverflow: https://stackoverflow.com/questions/56467564/why-does-directly-calling-groovy-closure-not-work-but-using-call-does