A standard flat collar places contact force across a narrow section of the dog’s neck. That may be suitable for a small dog walking calmly on a loose lead, but a large or powerful dog can generate much more force when lunging, pulling, or stopping suddenly.
A wide fit collar spreads that force across a broader surface area. This can make the collar more comfortable, harder to slip, and better suited to strong dogs. The wider construction also distributes pressure more evenly and reduces concentrated strain on the collar and its hardware.