Why is distilled water used instead of tap water?
Of the two, distilled water is the purest because the water undergoes boiling that kills off most organic contaminants. Deionized water may still contain minute amounts of organic material in it which could affect the outcome of an experiment.
Also question is, can tap water be used instead of distilled water?
Yes, you can drink distilled water. However, you might not like the taste because it's flatter and less flavorful than tap and bottled waters.