All tap water contains dissolved lime to a greater or lesser extent. "Hard water" is the term used to describe water on the high end of mineral content. The harder your water, the more lime in the water.
Even "soft water" has some mineral content. When heated, lime solidifies as "scale" on contact surfaces. If you boil water in a kettle at home, you will no doubt have already noticed this buildup on the sides and bottom of the kettle.
This same buildup of scale happens inside the plumbing of your espresso machine. It might be "out of sight", but it won't be "out of mind" for long if you don't take action to either remove it after it's accumulated, or take steps to prevent it happening at all in the first place.
Just think of those narrow plumbing tubes inside your machine as your arteries, and the lime content in the water as bad cholesterol in your blood. Overtime, the arteries of your beloved espresso machine become blocked, its blood pressure rises, and before you know it your machine is in bad shape and setup for a "heart attack".
It is recommended that you regularly "descale" your machine with a product such as Cleancaf. By regularly, that means at minimum once a month, and more depending on how frequently you are using your machine. It's a time-consuming process, and never completely removes all the lime buildup.
An even easier and more effective method is to avoid the buildup of scale in the first place. The only way to do that is to use distilled water. Yes, it adds to your cost, but it will honestly extend the life of your machine by years, and save you valuable time by not having to do the regular descaling. The more expensive your machine, the more using distlilled water makes sense.
You could of course buy distilled water, but a cheaper option is to make your own at home. I personally use one of these Countertop Water Distillers which I have had for about 5 years now, and it's still going strong.
Anyhow, give it some serious consideration. There's nothing like preserving the life of your espresso machine, while at the same time eliminating an extra cleaning duty from your weekend list of chores.
