Water needs a destination
Downspouts should move roof runoff away from the foundation, sidewalks, window wells, and low spots where water can pool and freeze.
Every roofline is different
Long runs, valleys, steep pitches, and upper roofs can overload too few downspouts. A contractor should size and place outlets based on how water actually reaches the gutter.
Small changes can help
Extensions, elbows, splash blocks, and reroutes can solve many drainage issues without replacing the whole system.