Calcium chloride is a brewing salt added to water to boost calcium and chloride together — the counterpart to gypsum's sulfate-driven sharpness, contributing a fuller, rounder, maltier perceived mouthfeel instead, and a common addition for balancing out a sulfate-heavy hoppy beer's edge rather than letting bitterness read one-dimensionally sharp.
Sulfate-to-chloride ratio, not either number alone, is the real target
Most modern water-chemistry advice for beer focuses on the ratio between sulfate and chloride rather than either mineral in isolation — a higher sulfate-to-chloride ratio pushes perceived character toward crisp and hop-forward (Burton-on-Trent's own extreme profile), while a more balanced or chloride-leaning ratio (closer to Dortmund's dual-mineral water) reads fuller and maltier. Calcium chloride is the standard tool for shifting that ratio toward the chloride side without needing to reduce sulfate directly.
Calcium's other job, independent of chloride
Like gypsum, calcium chloride's calcium content also helps buffer mash pH and supports healthy yeast flocculation, a benefit that comes along with the chloride addition regardless of whether a brewer specifically wants more chloride character — which is part of why calcium sits at the centre of nearly every brewing-salt discussion regardless of which specific flavor direction (sulfate-forward or chloride-forward) a recipe is chasing, and why most water-adjustment recipes include at least one calcium-bearing salt no matter the target style.