Good for when you're assembling something, and the instructions says to use a M5X20 screw and the screws aren't labeled, and you have multiple sizes. Now you can check it with this device and not be frustrated. Hope you enjoy.              
The holes are loose on purpose so not to wear them out. If you have to force the screw in, then it's the next size up. to measure the length just place the screw head in the notch as shown in the pics.