Simplicity, or perhaps more accurately apparent simplicity is the inevitable product of a strong understanding. Many of the things we think of as simple, or easy to use, are in fact mind-numbingly complex. The CPU in your computer comprises millions of transistors — literally too many to imagine, yet it is only one chip (albeit the most complex) amongst dosens each filling a specific purpose. All required for the machine's operation. On top of that hardware run millions of lines of code, each specifying a complex series of logical operations. Millions of lines in hundreds of discrete programs that have to work in unison.
You just click, drag, and push buttons. Simple. Anyone can do it.
Apparent simplicity therefore is hard work, you have to understand a task so thoroughly that most of the work can be done only once. If you get it right the user of your tool or system has only the essential variables to contend with, and knowing those variables should enable them to predict the method of use. Simplicity enables better learning, greater focus, and increased efficiency.
Despite all the benefits we are terrible at simplicity. We often have to complete tasks before we fully understand them; we learn as we go, we improvise, we muddle through and we (often) succeed. This is admirable in itself, we don't always have the luxury of history and experience to draw on. If we could make more time for simplicity, however, find our understanding, and invest in better tools, then I believe we would reap the benefits the next time we were learning under pressure. Old familiar jobs would take care of them selves and we would be at liberty to simplify the next great work.