A milking machine actually runs two separate tube systems side by side: one carries milk from the teat cups to the collection bucket, and a completely separate one — the pulse tube — carries the rhythmic pulses of air that open and close the liner around the teat, which is what actually creates the milking action. They look similar but do very different jobs, and they're not interchangeable.
Because the pulse tube is constantly flexing as it carries those rhythmic air pulses throughout every milking session, it's one of the components that perishes with age and use — rubber and rubber-like tubing gradually loses flexibility and can crack, especially with repeated exposure to cleaning chemicals and sun if stored improperly. A tube that's lost its integrity disrupts the pulsation rhythm, which can slow milking down or, over time, affect udder health if the rhythm becomes inconsistent enough to stress the teat tissue.