While you’re trying to measure something, this feature allows the tape to latch onto the end of a nail or screw, simplifying the task of measuring flat surfaces. From there, you can hammer a nail or twist in a screw and hook the end of the tape onto it for an accurate measurement.
Rumor has it that the blue side can even remove some pen marks, too! So, next time you’re in the market for a new eraser, opt for the double-sided kind to account for different marks and paper types.