Servos are great fun, and incredibly useful when you want to make something move.
They do have some issues at times, though. Sometimes you need to set them to a specific angle for assembly. Turning the output shaft manually can be problematic since they're not made for that and on cheaper servos you can strip gears or break teeth pretty easily. If the servo already has broken teeth or such, it's motion can be erratic and it's best to know that before you put things together. Finally servos can vary a bit in what their pulse width bounds are. If you need a full range of motion from them you need to figure out and account for those bounds (Adafruit servo libraries allow you to set them when you construct a servo object).
All in all, it would be nice to have a way to fiddle around with a servo before building it into a project.
This project does just that. Hook up a servo and test it's range of motion (0-180 degrees), test the smoothness of its motion by having it sweep back and forth between 0 and 180 at different speeds, and try different boundary pulse widths to find the servo's limits.