It has become code here to install pressure tanks and check valves in houses.
The purpose of the tank is to allow for expantion in the lines.
The check valve will not allow water once it passes it to make its way back out to the water main at the street.(There have been several cases where one houses water supply becomes contaminated and then has been pushed back out to the city main contaminating the whole supply)
When all outlets are closed in the house and the water heater is heating water, the expanded water actualy pushes against the check valve (closing it) not allowing a room for expantion, this is where the expantion tank comes into play.
Water hammer arrestors are used where there is a quick closing valve (washers, some showers).