"What is this slack in power lines? What purpose does it serve?"
A: "Former CATV tech here. It’s called an expansion loop. Coaxial cable (for cable tv) is mostly aluminum which expands and contracts with temperature changes moreso than the steel supporting strand to which it is attached. If this expansion loop wasn’t there, the cable would literally suck out of the compression fittings in the Winter and kink in the Summer."
Where do you call it like that? I only know it as Schultüte.