When you pour hot water over coffee grinds the coffee good, flavorful stuff dissolves and is released more quickly than the acids. The smaller the coffee particle the more quickly the acids are released. Take a look at what a blender type grinder gives you... everything from dust to sand to practically unchopped beans. Pouring water over that means you don't get the flavor out of the big chunks but you are getting acid over the little chunks. By getting the more consistent grind of a burr grinder you can find the right size grind for your pot, getting all of the flavor but without all of the acid. There is a very noticeable difference.