Why in Javascript event handler functions with parentheses?

Javascript gurus, look at this code:

<button onclick="handler()">ClickMe</button>
        <script>
            function handler() {
            alert("clicked");
        }
     </script>

Why onclick event should be assigned to handler with () onclick="handler()"? In this case alert is called. But according to the logic described as answer to similar question https://stackoverflow.com/a/3247044/2543590 onclick assigned to result of function handler, not to function itself. I believe to assign onclick to function it should be like this

onclick="handler", 

but in this case alert is not called. Why?