Modern instrumentation is very accurate and reliable. But it’s not perfect and can go wrong, sometimes in ways which are not completely obvious. This is exacerbated by the fact that precision instrumentation is used in hostile environments, such as building sites, where rain, dust, handling and corrosive substances can affect your measurements. For example, dropping an accelerometer on the ground may break a crystal – the device will still have an output, but it may not be as linear as it was when delivered. A calibration will allow you to check this and have confidence going forward. Think of it as a kind of MOT test for your instrument.