Using AddEventListener Or Predefined Events (OnClick / OnLoad)

I have been reading and practicing Javascript for the last month and so far I'm happy with it. I noticed that you can add Event Listeners and trigger a function based on that event but I also noticed that you could add an event directly to any element as an attribute, something like:

Code:
<p onclick="doSomething()">Click Me</p>
function doSomething(){
//do something
}

So my question is why would someone add and event listener instead of adding that event directly in the element (as the sample above)? The reason I'm asking is because adding event listeners involves more code:
Code:
var elementName= document.getElementById('elemenstsID');
elementName.addEventListener("click", doSomething, false);

I guess what I don't know understand is why would someone choose to add an event listener instead, I know it is more OOP but doesn't the "onclick","onload" etc., do the same thing?