When 0 is used as a placeholder, it allows people to write numbers in shorthand and tell them apart. For example, the number 509 is shorthand for "5 times 100, plus 0 times 10 plus 9 times 1." Since zero adds no value to the 10's place, it can be used as a placeholder there, creating the possibility of the shorter notation.
Without placeholders, there would be no way to write numbers in shorthand notation, and also no way to tell them apart. For example, without 0 as a placeholder, the numbers 59, 5009 and 5090 cannot be written in short form.
When 0 is used as a leading 0, it can safely be removed. For example, in the number 0509, the first 0 can be removed without changing the value of the number while the second 0 cannot because it is a placeholder.