Much like building materials and techniques have changed over the years, plumbing systems have incorporated new technologies. Engineers have designed more efficient systems which incorporate the needs of current population numbers along with water supply problems and waste disposal considerations. Modern plumbing fixtures use less water at higher pressure for conservation and piping is made of cleaner materials designed to work with the new style with less maintenance issues.
The old plumbing systems used copper pipes for incoming water and steel for wastewater return. The system would last forever if used although if a house sat empty the pipes could corrode without the regular water pressure of daily use. Steel rusts over time, but the heavy pipes were designed with the knowledge of how long it would take to become a problem and they rarely need replacement when used regularly. When a clog occurred, it was simple to clean with chemicals or physical means. The system worked well until cleaner, more eco-friendly systems were designed.
New standards of green building and water efficiency have led to the design of low water use appliances and fixtures. The fixtures incorporate a design which uses water at a higher pressure to conduct the same job which once required a higher volume. Washing a greasy pot, for example, can be performed with a constant stream of high volume water or a quicker stream of water with a higher pressure to perform the same amount of work. Pipes are built of cleaner materials, plastics which can't corrode or wear down over time while maintaining their strength for the pressure of water pouring through them.
How to Make the Two Work Together
The two systems are interchangeable with the right adapters installed according to the experience of a licensed plumber. Any renovations or upgrades to an older house should include switching to the newer system as appropriate for the project. A professional inspector can further assess the system for proper function and recommend a replacement plan to bring it up to modern code and expectations. Although it never hurts to have a new house independently inspected outside of the contractor's plumbing license and building inspector's approval, older houses especially need to be thoroughly inspected so you'll know what you're getting into before the purchase. Older houses also need ongoing inspections to assure efficiency and keeping up to modern standards.