Measuring by volume is quicker and requires no equipment. It is also the largest single source of inconsistency in home baking, and the reason is that volume does not describe how much material is present.
Packing changes the weight in a given cup
Flour and similar powders trap air between particles, and how much air depends entirely on how the cup was filled.
Scooping directly from a bag compresses the flour against the sides of the cup, while spooning it in loosely leaves it aerated, and the difference between the two is substantial.
Since the recipe was written by someone using their own filling habit, a reader with a different habit is working from a quantity the author never intended.
Humidity and storage shift the same ingredient
Flour, sugar and salt all absorb moisture from the air, and a damp week leaves them heavier and more compacted than a dry one.
A cup measured in humid conditions therefore contains more water and less flour than the same cup measured in dry conditions.
Weighing removes the packing variable but not the moisture one, though the remaining error is far smaller than the one it eliminates.
Ingredient form matters more than most recipes admit
A cup of chopped vegetables depends on how finely they were cut, since smaller pieces pack more closely and leave less empty space.
Brown sugar is conventionally packed and white sugar is not, which is a convention a reader has to already know because the cup itself carries no such instruction.
Grated cheese, herbs and anything irregular are worse still, and these are the ingredients where volume measurements diverge most between two careful cooks.
Ratios are what actually make a recipe work
Baking outcomes depend on the proportion of flour to liquid to fat to sugar, and those proportions are only meaningful if each is measured on the same basis.
Weight allows a recipe to be scaled cleanly to any quantity, whereas scaling volumes compounds each measurement's error at every step.
It also allows a recipe to be described as a ratio at all, which is how a cook eventually stops needing the recipe.
The practical gains are speed and fewer dishes
Ingredients can be added successively to one bowl on a scale with the display reset between each, so several measuring vessels are never used.
Sticky ingredients weigh straight into the bowl rather than needing to be scraped out of a cup, which removes both waste and washing.
The result is that the more accurate method is also the faster one, which is unusual and is why professional recipes are written in weight throughout.