This program is targeted towards system fonts, their sizes, and their font smoothing minimum size. The MacOS systems only allow a select range of fonts to be used as system fonts. This is the overall font that you see in the titles of windows, menus, dialogs, etc. Font Smoothie will allow you to change the system font to any font that is available in your system. It also allows any size font.
Some fonts look better bigger, others smaller, depending on the situation and font design.
You can now change your views font from Font Smoothie as well. This is the font that displays underneath the icons in the Finder. This can also be changed by the Appearance control panel. But if you're like us, you hate waiting for the Appearance control panel to load, and it is much nicer to change all fonts from one program. A restriction exists that only allows the font size to be 12 point or higher.