Performing the calf raise on one leg at a time doubles the load each calf handles compared to a standing two-leg version, since your full bodyweight plus the dumbbell rests on a single foot. Holding a dumbbell for extra resistance and balancing on one leg builds calf size and ankle stability at the same time. It's an efficient way to address strength imbalances between your left and right calf.
Log your dumbbell single leg calf raise sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.