The main goal of PEMDAS is let you work with formulas. Simply type in an equation, hit enter, and you have the answer! There isn't any messing around like the one-step-at-a-time calculators, where half the work is just trying how to figure out how to enter the equation.
PEMDAS lets you assign variables by using the equals sign on your keyboard. Like the auto-cached answers, variables show up in the variable cache for easy one click inserting into an equation.
PEMDAS keeps track of everything you've calculated. The formula cache gives you one click access to recalculate the equation. To clear the caches and reset everything, hit Clear (or Shift+Delete) on your keyboard twice, or double click the clear button on the widget.
The answer cache makes it easy to see all your answers, and keep track of the variables you've used. The result of each calculation you do is assigned a unique variable name (ans1, ans2, ans3, etc.), and the previous answer is always stored in the 'ans' variable. To clear the caches, hit Clear (or Shift+Delete) twice real fast.
PEMDAS can display numbers in floating point notation, scientific notation, engineering notation, and as a percent. Additionally, in programmer mode, you can display numbers in binary, hex, and oct.
PEMDAS is color coded in groups according to what the buttons do. Hopefully this helps with distinguishing everything apart.
PEMDAS is a good alternative to MATLAB, Mathematica, and Maple, for basic calculations. It's a little overkill to launch those programs to do 3*41+22. And you can always copy the formula to paste into those programs if you need to.
PEMDAS has a special mode just for programmers. It will let you enter hex, binary, and octal numbers, and format the answer in those. And the Programmer mode is just a layer on top of the regular mode, so you can do all your programmer stuff side by side with regular calculations, like 2*2+sin(4), without having to switch back.