EverFont is a utility that enables you to preview the font styles that are being installed on your computer. This type of programs might come in handy to graphic designers who are unsure which text styles to use in their projects and want the preview their texts using with certain Mac fonts.
The tool provides you with a straightforward interface where all you need to do is to select the proper font style and enter the text content in the panel from the bottom side of the main screen. You can alter the font size if necessary.
There's also a Pro edition of the program. Still, the only differences between the two editions seem to be their price (one is free, the other comes with a price tag) and the fact that they have different versions.
However, you already have a solution for previewing font styles on your Mac. The program is called Font Book and enables you to preview text styles using your own words as samples. The feature that's available in EverFont and is currently missing from Mac's default tool is the ability to export the data from the preview screen and save it as a PDF file.
So, since not even the Pro variant of the tool doesn't bring you more features, I think you should stick to using Mac's Font Book to preview text styles.