Hello. I am a bit confused about function calls. Why dont we use parentheses when we call the other functions as a method of the object, e.g when creating event handlers?

PHP Code:
document.getElementById("toolbox").onclick=serto
And why we do use parentheses when calling the function as the attribute value?

PHP Code:
<div id="toolbox" onclick="serto();">

</
div
In short I wish to learn how to call functions the right way?

Thanks in advance.