Filtering water isn`t a necessity when brewing coffee, though unfiltered water can alter the taste. If you`re dealing with hard water or water from a well, there could be a lot more minerals in there to deal with. For the purest coffee flavor, filtered water may be a necessity for some people, depending on their home water supply. There are also some coffee makers that include an inbuilt water filtration system that takes care of this process for you, giving you a great cup of coffee every time.