Why is checking button click in Unity done this way?

I find the Unity way of writing the GUILayout.Button in onGUI to check for button click to contract the way I am used to seeing UI code, in which you instantiate a UI object and set up a click event handler for it. What's the reasoning behind doing it this way (from a framework creator's viewpoint)?

 function OnGUI {
     if (GUILayout.Button("Play")) {
         // button clicked  
     }
 }