Gages are indirect reading instruments. The measurement units live not on the scale but off-site (for example, in a calibration room), and a master or other standard object acts as their substitute. The workpiece is compared against the master and only indirectly against the measurement units. The gage thus evaluates not the dimension itself, but the difference between the mastered dimension and the workpiece dimension. Examples of these comparative gages include an adjustable bore gage, mechanical snap gages or bench stands with dial indicators.
Gages tend to be faster and more accurate than measuring instruments. This is because they are dedicated to a particular size, and over their short range, they produce higher resolution and hence, greater accuracy.
Source: When Gaging Becomes Measuring That Becomes Gaging