Rapid-Pi was designed with a single purpose in mind - to save you time when editing math in documents. Rapid-Pi's text-based input is a simply faster way to input math.
Most equation editing programs require you to click on toolbar buttons or go through menus in order to insert symbols and expressions. This process is inherently slow and painful. Even simple things like fractions take up too much time - you have to click on the fraction button, then click in the numerator, type the numerator, click in the denominator and type the denominator. Consider how much faster and easier it would be if you could simply type "1/4". That's exactly what you can do with Rapid-Pi.