Park Tool FFG-2 Frame & Fork Alignment Gauge Set
Improperly aligned frame or fork dropouts can cause bent axles and quick release skewers, as well as tracking and shifting problems. The FFG-2 quickly determines whether rear frame or front fork dropouts are aligned and parallel.
If an error is detected, long handles and comfortable molded grips afford plenty of leverage for straightening. These Park Tool gauges adjust to compensate for various hub widths from 100 to 160mm.
Probably not a tool that even super avid hobbyist would consider. If you're into steel frame restorations with modern hub width wheels, then yes, buy this set. If you have strictly aluminum or carbon frames, then mostly, these things will tell you whether or not your stays are bent or broken and in need of replacement.