We do recommend a thermal expansion tank for those who want to extend the life of their new water heater(s). In the future, we expect expansion tanks will be required by city code. Expansion tanks are a small additional cost that will save the homeowner money in the long run. The expansion tank helps reduce stress on valve stems, safety valves, and ball seals, and it is a pressure safety component.
As of April 16, 2015, new energy efficiency regulations have been put in place by the Federal government, which have resulted in a 20-30% increase in water heater prices. Warehouse supplies of older water heater units were expected to be depleted an estimated three months after the regulatory date.
Third party YouTube video that explains the importance of a thermal expansion tank.