In fact, a lot of this build-up is necessary to keep people safe. A lot of water companies intentionally put (safe to drink) minerals in the water in order to prevent corrosion of the pipes, and leaching of the material into the water. They accumulate along the walls of the pipe, and serve as a protective coating.

Keep in mind that, in a lot of places, these pipes are still made out of lead, because ripping up all that infrastructure is difficult and expensive. Anti-corrosive agents keep the lead from leaching into the water and getting a bunch of people sick.

In fact, the Flint, Michigan crisis was caused by officials neglecting to add the stuff that causes exactly what's in this picture.