Volume changes with packing and ingredient shape
A cup of flour can hold different weights depending on how it was filled, leveled, sifted, or compressed. Salt crystal size and starter bubbles also make volume conversions inconsistent.
Those differences affect hydration and salt percentage directly. A small measuring error may be hard to notice in one loaf but becomes larger when the recipe is multiplied.
Weight supports direct percentage checks
With grams, hydration is simply water weight divided by flour weight. Salt percentage is salt weight divided by flour weight. The units cancel, which makes the ratio easy to audit.
A scale also lets you tare the bowl between ingredients, compare the measured total with the calculated total, and divide dough into equal portions without relying on visual estimates.
Use suitable precision
A normal kitchen scale is usually adequate for flour and water. Very small yeast or salt quantities may need a scale with finer resolution, especially when making a small batch.
Unit conversion does not create precision that was not measured. Converting a rounded ounce value to grams can look exact while still carrying the original rounding error, so keep the original measured unit and precision in your notes.
Related calculators
DoughMath guides are educational explanations for formula math. Use the linked calculators for exact weights before baking.