Reward, and a meal.
Carrying their food means the day can run longer than you planned.
Most of training is timing, and most of timing is spent reaching. Clipped to the lead, it closes that gap - a small weight against your wrist, a zip pulled with one hand, a reward given while the moment is still warm. And on the days that stretch, when the walk becomes a morning and the morning becomes an afternoon, it holds enough of their own food to see them through it. One glance at the clear side tells you what's left. Zipped shut, the smell stays with the food rather than with you. Over weeks it stops being a thing you remember to bring and starts being part of how you leave the house: keys, lead, this. Your dog learns the sound of it. That is usually enough.
