When you steam, you lose water, but not minerals. So the mineral content in the boiler builds up. You also can get some leached metals. The result is a brackish taste. This is not a factor in the coffee taste, since the espresso water passes through the HX, not the boiler.

However, mineral build up can cause scale and will spoil Americanos. Once a week to once a month (more frequently if you are in a hard water area), fill your tank with distilled water, and open the hot water tap. This will drain the boiler and replace the highly mineralized water with distilled water. This is called "blowing down" the boiler and should be done routinely with espresso machines.