Question

why are standard unit used in measurements​

Answer 1

Standard units are used in measurement because other units such as hand span are not appropriate because each person has different height.

Answer 2

We need standard unit for measurement to make our judgement more reliable and accurate. For proper dealing, measurement should be same for everybody. Thus there should be uniformity in measurement. For the sake of uniformity we need a common set of units of measurement, which are called standard units. Nowadays Si units in science and technology almost universally.