It's aimed at those who need only the basic features, as well as those who need to do more advanced operations. Different settings (called modes in the application) are what makes this tool so versatile.
GCalcTool has four different modes: basic, advanced, financial, and scientific. The basic mode is aimed at users who don't need any complicated features. The advanced mode adds a few other operations, like fractions and square root.
The financial mode includes more advanced operations, related to the financial world. Finally, the scientific mode offers all kinds of features, different units, numbering systems, and other advanced options.