Water

more than one reason.

distilled watter is more pure. It has nothing in it at all. It will keep your creedo/beads/oasis/whatever clean.

it also has no ordor or taste. A few months ago the Akron water supply had a HUGE alge problem. THis caused all the water in akron to smell very bad. If this water would have been used in a humidor it would have made the humidor smell bad and the cigars would pick up this taste also.
 
Tap water is a sure fire way to ruin not only your humidification device but your humi too. Tap water (as Matt said) contains all kinds of stuff that will eventually clog the poors in the grain of the wood and keep it from absorbing and releasing humidity.
 
And Distilled H2O is CHEAP! I pay only 50 cents at the grocery store for a gallon.
 
The same reason you wouldn't use tap in a steam cleaner, clothes iron or a radiator.

Tucson has really hard water. Mineral deposits galore. My Oasis would end up crusty if I used tap.
 
I remember the water in Phoenix. YUCK! And yeah, I used distilled in everything, especially the iron.