Why do power lines have a slack?

Overhead lines have 'sag' (not 'slack') because the conductors need to be able to expand and contract as the temperature changes. If sag isn't built into the design of the line then, when the conductors contract, they will put undue lateral strain on the supporting poles or towers. It is important that, with maximum sag, the minimum clearance distance between the line conductors and the ground below is always maintained. This safety clearance increases, of course, with line voltage. Allowing for the necessary sag part of the design-process for any overhead line, and achieving it becomes the responsibility of the linesmen when the line is eventually constructed.