Tube liners are critical components used within brass push-fit plumbing pipe and fittings systems. They are designed to be inserted into the end of a pipe, providing essential internal support. This reinforcement prevents the pipe from deforming or collapsing when a push-fit fitting is secured, ensuring a reliable and leak-free connection. Tube liners help maintain the integrity of the pipe's internal diameter, which is vital for optimal flow and long-term system performance.