You can't easily change $20.00 to $200.00. You could try changing it to $20.000.00, but that would probably overflow whatever box it's written in, since the amount is usually right-justified to begin with.
The decimal point protects the right edge of the amount, while the dollar sign protects the left edge.
Similarly, when people write out a number in English on a check, they often write "Twenty dollars only", not just "Twenty dollars", in order to prevent other words from being added to the end, e.g. "Twenty dollars and a hundred". (There's a nursery rhyme that goes "four and twenty blackbirds", so writing the smaller number first is clearly not unheard of.)
The decimal point protects the right edge of the amount, while the dollar sign protects the left edge.
Similarly, when people write out a number in English on a check, they often write "Twenty dollars only", not just "Twenty dollars", in order to prevent other words from being added to the end, e.g. "Twenty dollars and a hundred". (There's a nursery rhyme that goes "four and twenty blackbirds", so writing the smaller number first is clearly not unheard of.)