Although using a gram scale is often associated with snobbery, the advantages of using one outweigh these kinds of snide remarks. In the end, the primary reason for using a scale is that it allows consistency.
Mass > Volume
Physics tells us that when it comes to measuring things, mass is much more accurate than volume. This is particularly clear when measuring flour. Many baking recipes clarify whether the flour is supposed to be sifted or not; this is because sifted and unsifted flour take up different amounts of space. Weighing by volume, then, not only leads to possibly using too much or too little flour, but it also means that every time the recipe is made, it is unlikely that it will be exactly the same.
This is also true of coffee. All coffee beans are not created the same way—they are different shapes and sizes. And, different ways of brewing coffee require that you use different grind levels, which, obviously, take up different amounts of space. These different grind sizes do not pack the same way. When measured volumetrically, smaller grind sizes have smaller air pockets than do larger grind sizes. Therefore, because mass is a much more accurate way of measuring things than volume, a scale is going to be a necessary tool for brewing coffee properly.
A Gram Scale Allows Experimentation
On top of all of that, using a scale also allows you to experiment in a better fashion. If you do not know how much coffee or water you are using, you will not be able to adjust certain elements to improve your coffee tasting experience (e.g., grind size). Having a scale helps you to hone in on your favorite water to coffee ratio, and to be able to recreate that same cup every time. With that said, the perfect water to coffee ratio is much debated, so it will be something that you have to play around with. Personally, I use a 16:1 ratio—16 grams of water to 1 gram of coffee. So, for example, if I am making one mug of coffee, I might use 15 grams of coffee and 240 grams of water.
Experimentation also extends to roasting coffee. A gram scale is absolutely necessary for roasting coffee. Using a gram scale, and keeping a log, helps to improve your roasting skills and get better roasted coffee. One way that a scale is necessary is that it allows you to recognize the coffee’s percent weight loss, which correlates especially so with the degree of roasting that you use.
The above-picture is the current scale that I use when making coffee. Although it has been especially helpful, I would recommend a gram scale that weighs to tenths of a gram (like this one by American Weigh). This will allow even more exact measurements. Until I get one of those, my method now includes weighing coffee beans in such a way that just tips the scale, so to speak, to the next number. For example, if I want 15 grams, I pour quickly to 14 grams and then add one bean at a time until the scale changes to 15. Doing it this way allows me to get closer to 15, rather than getting 15.9 and never knowing.
All this to say, As James Freeman says, “If you’re going to buy good coffee and attempt to make it well, why be approximate?” This truly is the essence of why you should use a scale. If you want to have consistently good coffee, you should use a gram scale.