With each coffee maker, their filtration method changes. While some have stainless steel, other would have a nylon, or some would have paper filter and another would have a cloth filter. Each of these filter materials not only change the profile of coffee but also require a special grind size as their perforation size differs. Also, the grind size for each coffee differs for each filter. We do this mostly to bring out certain flavours we wish to highlight in each coffee. While brewing a Monsooned Malabar in a Chemex, the grind size would be in between the range of medium and coarse and we would get a sweeter cup profile that also appears to be a golden-ish colored cup. However, when brewing the same coffee, Monsooned Malabar in a French press, we would keep it coarser than other coffees because while the earth notes would be more prominent anyway, we don’t want them to overpower the inherent sweet taste. Emphasizing on personalising each cup of coffee, we could add more coarsely ground powder, meaning increase dosage, to a chemex, to also bring forth the earthy notes and a fuller body. Concluding, grind size also helps us play within a range to get different flavours and tastes out of the same coffee and the same coffee maker.
If you order a medium or a finely ground powder and brew in a French press, there are chances the powder would escape the perforation of the filter and escape into your cup. Hence sometimes your French press coffee would also be over extracted, cloudy or bitter. Hence, while buying coffee, always specify the coffee maker you would be using.