+=的理解问题

<p id="demo"></p>

<script>

var x = document.getElementById("myBtn");

x.addEventListener("mouseover", myFunction);

x.addEventListener("click", mySecondFunction);

x.addEventListener("mouseout", myThirdFunction);

function myFunction() {

    document.getElementById("demo").innerHTML += "Moused over!<br>"

}

function mySecondFunction() {

    document.getElementById("demo").innerHTML += "Clicked!<br>"

}

function myThirdFunction() {

   document.getElementById("demo").innerHTML += "Moused out!<br>"

}

</script>

上面代码中，+=我的理解是document.getElementById("demo").innerHTML = document.getElementById("demo").innerHTML+"Moused out!<br>"；

//设置P标签中的内容，但是我不理解设置document.getElementById("demo").innerHTML有什么作用

这里有什么作用？？怎么理解，刚学的菜鸟，求解答~谢谢