You don't need it at all. Some of us just like to have them because we tend to be picky perfectionists about the things we do. LOL
The 1588 is preset for 100 degrees. They are fairly accurite. 1 thermometer is enough otherwise you drive yourself insane.
Those are the most very basic incubators. They come with nothing extra. It is a styrofoam box with the heating element and the thermostat you have to regulate yourself.
You will be much happier if you save your money and buy direct from GQF , the makers of Hovabator , and get a Genesis 1588 ...