Like all other forms of languages, mathematics developed because of the necessity to communicate ideas. In the case of numbers, it was to express and record ‘countables’. Below is a little video that can explain this to pupils in the primary grades. Its title is History of Mathematics but I believe it should be titled History of Numbers. There’s no mention of the Greeks and the mathematics they developed.
Our invention of numbers and number system shows us that we are inherently problem solvers ad that we invent symbols and systems to express and communicate our ideas. Mathematics is the most powerful language humans have invented.