Companies that repipe houses

"Repipe specialists". "Change out those old and rusted pipes." "It can often be done in one or two days." The ads show brown water coming out of shower heads. "Right now we'll throw in a free tankless water heater." At least a couple companies are advertising this heavily in my area. I wouldn't think this is a simple project. I would think it would require ripping out a lot of walls and not something I would consider unless totally, absolutely necessary. What is their angle here?