One hundred grams is not half a cup by default. It is about 0.83 cups of all-purpose flour, 0.51 cups of granulated sugar and 0.44 cups of butter. Cocoa powder is much lighter, so 100 grams occupies more than one cup.
When a search result says that 100 grams equals a particular fraction of a cup, verify that the result names the ingredient. A weight-to-volume answer without an ingredient is incomplete.
A 100 gram quantity appears frequently in European baking recipes, chocolate recipes and scaled ingredient lists. A digital scale lets you follow that amount without rounding it to a difficult cup fraction.