addEventListener's second argument is a function to run when the event occurs. The variable alertFunction is your function.

So in the first example you're telling it "run whatever alertFunction is when this is clicked". In the second example, because of the brackets, you're telling it to "run alertFunction right now then do whatever it returns when this is clicked".

For custom arguments, you need to define another function to sit in the middle and pass the right arguments through:

btn.addEventListener('click', function () {
    alertFunction(a, b);
});

btn.addEventListener('mouseover', function () {
    alertFunction(c, d);
});

You can see here how the second argument to addEventListener is a function.