It doesn't; it affects the cost of laying it. The cable must be anchored to the sea floor at regular intervals and it's cheaper to place an anchor at a few hundred meters depth (the coastal shelves) than at 3 km (depth of the Biscay abyssal plain).
It wouldn’t surprise me either if it helps in some legal situations - if someone runs over your cable in Spanish territorial waters vs international waters for instance, it’s probably easier to get paid back.