Tyre depth is important for two reasons: 1 Bad tyres are dangerous when you don't expect it ie. breaking in emergencies and wet surfaces. Good grip is important and the only way to measure your tyre reasonably accurately is by using a gauge. This particular gauge is easy to use and pretty accurate and needs to be used only twice a year.
2 In Autumn the weather turns to be very wet and slippery if not ice and tyre depth becomes important because no grip makes you lose control . Thus a depth gauge is a good precaution.