I think the thing you didn't realize is that the drop checker is supposed to have 4dkh distilled water inside it. That is why it's not the same as checking your KH/GH and PH ratios of your tank. That is how you are assured a certain CO2 level. As Bert said, it's not mandatory, but it sure is helpful. At least you know what CO2 you have in your tank in that area.