As the name suggests it is a tool to straighten your filament. "But.... why?" you ask?
Well, personally I needed straight filament to load my filament buffer. But this is not the only time you can use a straight filament: You can use it to get a straight stud for friction welding or to align the bontech gears of your printer or you have troubles loading your extruder with bent filament or you simply like cool tools.
And here is a video of the tool in action:
Please ignore the audio book. (tip: "Lord of all things" by Eschbach, it is very good)