Here’s my argument for baking with a scale:
1. It’s more precise, especially for flour. A cup of water at my house will be about the same as a cup of water at your house. But, a cup of flour at my house and a cup of flour at your house will most likely not be the same at all. Why? Maybe your flour is fresh from the grocery store and my flour’s been sitting in my pantry for 6 months. As flour ages, it looses moisture, becomes lighter and less dense, and absorbs more liquid. So, one cup of my flour will have less flour in it than one cup of your flour. Or, say I brought my flour home from the store and tried to fit the whole bag into my tupperware flour container. It wouldn’t quite fit, so I banged the container on the counter a few times to make more room. My flour is now much more dense and more compact than your flour, so one cup of my flour is going to have more flour in it than one cup of your flour. Ok, you get the point. If we both work in grams (weight) rather than cups (volume) we don’t have to worry. 100g of flour is always 100g of flour.
2. It allows you to flawlessly replicate something you’ve made before. Never mind making sure my cup of flour is the same as your cup of flour, what about making sure that every cup of flour I measure at home is the same as the last? Same problems here. Sometimes I have fresh flour, sometimes not. Sometimes I’m a little heavy handed when I scoop, sometimes a little light handed. Ever revisit a recipe only to find that what was once magical is now average or a total flop? Or, have you doubled a recipe and found it’s just not the same? It’s not always your imagination. Especially in small batches (which is everything you make at home), a little too much or too little of anything can make a big difference in the final product. Doubling a recipe measured in volume will sometimes double a problem, whereby something you don’t even notice in the single sized batch is a big problem in the double.
3. It’s cleaner and easier. If you invest in a little kitchen scale you can measure all your ingredients in one bowl. Baking minus all the cups, teaspoons, pyrex measuring jugs, spatulas to scrape sticky things out of aforementioned cups, etc., etc. = less dishes for you. Just put your bowl on your scale, zero the scale, measure the first ingredient, zero the scale, measure the second ingredient, and so on. Kitchen scales sell for less than $50 for one that measures up to 10 lbs and down to 2 gram increments, or around $20 for up to 5 lbs and down to 5grams. I recommend the first, but depending on how much baking you do, you may want to go with the second. See here for a some options.
Converting grams to cups and teaspoons is tricky business because every different volume of every different ingredient will have a different weight. So 1 cup of milk may weigh 250g but one cup of flour may way only 120g. See here for a conversion chart that covers the most common baking ingredients.
Are there any exceptions? Sure. For the most part, I don’t work in grams when I cook because cooking is so much more fluid and less precise than baking. I like to taste as I go, and adjust the saltiness, sweetness, acidity, and spice according to the flavor of my dish and my ingredinets. Some lemons are more acidic than others, some tomatoes more sweet than others, etc. Cooking, unlike baking, is never about a precise formula, but rather about balance and layers. I also never work in grams when I bake what I call “grandma recipes.” Not necessarily recipes I get from my grandma, though there are some, but just small batch simple stuff that’s been passed down from somewhere. When I asked my grandma about making her famous cinnamon buns, she told me “just add the cream and mix it until it looks right.” I like to be precise. I like to know how things work and rise and bake and caramelize. But sometimes, I also like to not know how, to leave a little up to chance, and to just mix until it looks right.