After sediment is removed from Champagne, why can winemakers add a little sugar-wine mix even to dry Brut styles?

Answer: To balance sharp acidity and house style

To turn the wine into dessert wine — Dessert-wine sweetness requires much more sugar than Brut allows. The same ingredient can play different roles at different amounts: a tiny seasoning in a sharp sparkling wine, or obvious sweetness in a much sweeter style.

To restart the bubbles in the bottle — The main bubbles are already created by the second fermentation before disgorgement. Trying to restart fermentation after that would bring back sediment and pressure-control problems, which the finishing steps are meant to avoid.

To balance sharp acidity and house style ✓ — The small addition is a final tuning step, not a trick to make a dry wine taste sugary. Brut Champagne still stays below the sweetness range of dessert wines, and the sugar can soften high acidity while keeping the wine crisp.