3D printers are great. And if you are just printing little statues of Mickey Mouse, Yoda, or frogs to sit on your desk then chances are the dimensional capability of your printer is probably close enough to give you a decent looking model.
When you'd like to start building more complicated objects with your printer the dimensional capability becomes more important. The spacing between holes for instance may determine whether you are able to bolt it to another part.
Printing a calibration block is a good way to get your feet wet on a printer you haven't used before and will set a good foundation for future printing, knowing that the printed size is correct.
Step 1: Selecting a Calibration Block to Print - The Universe of Thingiverse!