Why grind coffee different coarseness?

Different methods of brewing coffee need different textures of ground beans, so it depends on the machine you're using, or the type of drip device. Refer to the operation manual for your machine, or for your grinder, or else next time you buy coffee beans, tell the store staff what you're using to brew the coffee and ask their advice about texture. If you buy vacuum-packed beans in a package, there might be instructions on the back. You could also ring or email a coffee importer or retailer; they'll be able to help.