Because this frequently changes when you change the scope by calling a new function, you can't access the original value by using it. Aliasing it to that allows you still to access the original value of this.
Personally, I dislike the use of that as the alias. It is rarely obvious what it is referring to, especially if the functions are longer than a couple of lines. I always use a more descriptive alias. In my examples above, I'd probably use clickedEl.