The increased pressure from when the water is heated can put pressure on the boiler and pipes, leading to wear and burst pipes. Adding an expansion tank provides somewhere for that extra pressure to go, thereby increasing the life of the tank and pipes.
Expansion tanks are fairly inexpensive and fairly easy to install. They can protect against expensive damage caused by excessive pressure in the boiler.
Costs vary greatly by region (and even by zip code). To get free estimates from local contractors, please indicate yours.