A map is a representation of a part of the world that is presented at a convenient scale to fit on a piece of paper of a suitable size to be conveniently usable by the map reader. Say for example to fit a map of Tasmania onto an A2 sheet of paper you would probably use a scale of about 1:1,000,000.

Image of 1:100,000 Scale Bar
Source: http://rsteward.napsk12.org/map-skills

What does it mean?

So what does a scale of 1:1,000,000 mean, and why use this scale over say 1:1,123,986?

A scale of 1:1,000,000 is a simple ratio (or fraction), which means that one unit on the map represents 1 million units on the ground. In other words 1 centimetre measured on the map represents 1 million centimetres, or 10 kilometres, on the ground.

Australian Population Distribution in 2000, Source: http://sgrhs.unisa.edu.au/student/prep_rural_prac/1-major_issues.htm

To answer the second question about the preference of scales such…