Why use only distilled water to humidify the humidor?

Distilled water contains less germs and free particles than tap water. More importantly, distilled water is free of lime, whereas tap water can contain quite a lot of lime, depending on the area. Tap water can leave permanent limescale deposits on the inner walls of the humidor and inside the humidifier, especially in the pores of the sponge element of the humidification system, eventually clogging it. Special humidifier fluids for humidors contain mostly distilled water as well as odourless antibacterial additives.