Why shouldn`t I use distilled water in my iron?

Distilled water has been identified to be more detrimental to the ironing system when used in isolation. It actually promotes corrosion more than tap water! The reason in simple terms is that distilled water is effectively looking to replace the natural minerals that have been distilled out and therefore attacks metal surfaces more aggressively.
Distilled water also has a different boiling characteristic to tap water. A cold refill on hitting a hotplate at high temperature may give a greater degree of spitting.
For these reasons distilled / purified water is not recommended to be used alone. However for very hard water areas there is a balance - we recommend that a mix of 50% tap, 50% distilled water can offer an improved situation.