Nominal Scales of Measurement
At the first level of measurement, numbers are used to classify data. In fact words or letters would be equally appropriate.
Say you wanted to classify a football team into left footed and right footed players, you could put all the left footed players into a group classified as 1 and all the right footed ones into a group classified as 2. The numbers 1 and 2 are used for convenience, you could equally use the letters L and R, or the words LEFT and RIGHT to label the groups of players.
Numbers are often preferred because text takes longer to type out and takes up more space. Another example is blood groups where the letter A, B, O and AB represent the different classes