Why weigh?
A scale tells you how much you actually have. Cups and spoons change with how you fill them, so the same written recipe can land heavy or light without you meaning to change a thing.
Cups are not a single weight
A cup of flour might sit anywhere from about 110g to 150g depending on whether it was sifted, scooped, or packed. Over several cups that gap adds up enough to change crumb, rise, and how wet the dough feels.
Grams stay put. If you need 400g of flour, you can stop at 400g. You are not decoding how tightly the cup was packed last time.
Where the chart numbers come from
The weights in our chart are the same ones the converter uses—rounded for display, and aligned with references bakers already trust, including King Arthur Baking's ingredient weight chart. If your bag, brand, or old family recipe assumes a slightly different cup weight, your scale still wins for that bake.
Common ballparks people compare against: all-purpose flour about 120g per cup (books sometimes use 130g or 140g), granulated sugar about 200g per cup, water about 237g per cup. Treat those as starting points, not scripture.
If you only have measuring cups
For flour, many people use dip-and-sweep: fluff what is in the bag, dip the cup without pressing the flour in, level the top with a straightedge. It is still easier to misread than a scale, but it is more repeatable than random scoops.