Why?
Because if you have only one, you'll be in a few awkward moments. These first involve you wanting to write something down. However, your sticky note pad already contains important information on it. Now, you must make a choice.
Choice One:
You can tear off the current sticky on the pad and then write down the thing you wanted to write.
You get what you wanted on paper, no sweat right? But it's not that easy. You first need to put the other sticky note that was on the pad in a safe place. Or you need to transfer the information. This would require getting an entirely separate piece of paper or opening your computer to type it down. And this is before you manage to lose it entirely. (Sticky notes are easy to lose.)
Choice Two:
You remember the thing you wanted to write down and keep the pad as is.
Pros? You keep the information currently on the sticky note pad.
Cons? There is a high chance of forgetting the thing you wanted to write down.
Solution:
There isn't one. So sorry.