Scale builds up in cast iron soil stacks and in pipes that have carried construction run-off. Over years the effective diameter can halve, at which point everything blocks constantly.
The chain flail spins inside the pipe, knocking scale off the wall without cutting into sound pipe material. It is followed by jetting to flush the debris out and a camera check to confirm the result.
Descaling is also a required preparation step before lining a scaled pipe — the resin needs clean pipe wall to bond to.