I'd say it is about answering the question "How many?". It becomes natural to give a distinct symbol for each answer smaller than a given limit say 10. If you

I'd say it is about answering the question "How many?". It becomes natural to give a distinct symbol for each answer smaller than a given limit say 10. If you have symbols 1, 2, 3, .... 9 then it would be pretty obvious you would also want a symbol for zero. The positional way of expressing numbers is a separate more evolved development of course. In Roman numbers you had I, II, and III, ... So why didn't they have say '-' to symbolize less than "I"? Maybe they just expressed it with the word "none".