Try to imagine that you are doing a problem set about calculating the Round-Trip Time (RTT) from the your computer to a remote server. You have already figured out that the

RTT = transmission delay + propagation delay + processing delay + queuing delay
= packet size / transmit rate + distance / speed of light + 0 + 0
= 23445 / 100000 + 3000*10^3 / 299 792 458 + 0 + 0
= ???

At this point, you may find your bag and take out your powerful calculator, starting inputing this tedious mathematic expression, digit by digit, and pressing "=". Oh God, result comes out, finally.
Think about it. Why do we have to waste our time to input this long expression to calculators to get the result? No, we don't want to waste time!
Now, JiSuan is coming to release our poor hands. Once we get the expression, just select it, press a shortcut, DONE! The result is automatically calculated and inserted in our assignment document.

JiSuan is simple -- only one function, but we try to help all students get rid of a "real" calculator from our hands, especially CS students studying in computer network.

By the way, Ji-Suan means calculation in Chinese.