Serial monitor is very useful to display sensor values, calculation values other relevant program values especially for debugging purpose. Since Arduino Uno does not come with a display built-in, it will hard to figure out the variable or other parametric values during the execution of the code. Hence the Serial Monitor becomes the most important tool to display data in order to understand /know that the coded program is behaving well as a way to display information on the computer or to send instruction from the computer to Arduino .