Font Book, in OS X, can only preview one font at a time. Oh sure, you can click on any number of font files at the same time, but you then get a cascade of windows. Doable, but tedious to manage. Font Pilot enables you to examine whole folders of fonts at one time - though at the moment it's not working for me, which is another issue. But, when it works, it offers features not found in Font Book. Whether those features are worth $15.95 is a matter of individual preference.