Question

why does a measurement need both a number and an unit for its complete specification

Answer 1

Explanation:

It is meaningless for the number without a unit and a unit does not mean anything without a number

For example:

Let us consider drawing a line using scale

1) The measurement given is 7         [meaningless]

When there is only a number it will lead to a confusion whether to draw a line for cm or m.

Hence, the above mentioned statement is meaningless.

2) The measurement is 7 cm

Now it is easy for us to draw a line as it is mentioned to draw a line for 7 cm.