The expansion tank is a vital part of your car's cooling system and acts as an overflow reservoir catching the water/coolant mix that is forced from your radiator as it heats and builds pressure in the system.
As you run your car, the temperature of the coolant circulating through the water jackets naturally increases causing increasing pressure in the cooling system. When the pressure builds to a high enough point, the radiator cap allows that pressure to escape into the coolant reservoir through a small hose, called an overflow hose connected by the neck of the radiator to the expansion tank. Most of the coolant that is forced into the expansion tank returns to the radiator naturally after a few hours or when the engine is cold.
Without

expansion tanks

, cooling systems would explode under pressure or the coolant would be forced from the overflow tube on the radiator and leak onto the road.
If your find yourself having to replace coolant too regularly then check your expansion tank and the rest of your system for leaks. Finding a new expansion tank isn't difficult, as there's a great choice of

expansion tanks

for any car at Eurocarparts.com. Just do a quick search to find the one you need.