So, you only need a descaler/softener if you have what is known as hard water, which is basically just water with a lot of minerals such as calcium and magnesium. You can do a test that shows how many GPGs (grains per gallon) your water has. According to http://home.howstuffworks.com/question99.htm,
If your water tests at 1 GPG (17.1 mg/L) or less, then you have soft water. Water around 1-3.5 GPG (17.1-60 mg/L) occupies a gray zone between soft and slightly hard water and 3.5-7 GPG (60-120 mg/L) is moderately hard. Hard water is around 7-10.5 GPG (120 - 180 mg/L), and very hard water is above that.
So if it does turn out that you have hard water, then you look into descalers and softeners. The reason for this is that the calcium and magnesium come out of the water as scale, which builds up on the insides of pipes, water heaters, etc., to the point where they can become completely clogged.
A descaler is used to remove this buildup. They do this because they are acids and release hydrogen ions when dissolved in water. These ions, in turn, eat away at the scale by pulling at them with the charge. A softener is used to clean the water as it comes, disposing of the minerals safely. Water softeners are mechanical appliances that are put into your home's water system. They exchange the minerals for something else, usually sodium, in what's called an ion exchange. A diagram of a water softener is shown below:
This website provides more information about the exact workings of a water softener.
Hope this helps!