Consistency is the real benefit
A cup of flour is a range; 120 grams is a number. When the flour, sugar and butter are the same every time, the bake behaves the same every time, and any change you make to a recipe actually tells you something.
That matters most where ratios drive structure: bread hydration, pastry fat, meringue sugar, caramel stages. In those recipes a 20 g drift is the difference between a good result and a puzzle.
It is faster and leaves fewer dishes
Weighing is additive. Put the bowl down, tare, add flour, tare, add sugar, tare, add cocoa. Nothing gets transferred and nothing else gets dirty. Sticky ingredients such as honey, syrup and peanut butter go straight into the bowl instead of clinging to a cup.
Scaling becomes arithmetic
Halving "⅔ cup" is annoying. Halving 150 g is not. Once every ingredient is in grams you multiply the whole list by one factor, which is why bakery recipes are written by weight and often as percentages of the flour.