Isopropyl alcohol is a solvent that dissolves many oils, glues, and various sticky things that you're going to encounter in an art studio or craft room. The same properties that make isopropanol excellent for cleaning electronic equipment make it fantastic prep for getting a grease-free surface prior to painting.
You can use it to dilute the ink so that it takes on a lighter transparency, for a layer blending or glazing effect. More than that, though, rubbing alcohol is useful as a medium for your painting, similar to role that water plays in watercolor.
All rubbing alcohols are unsafe for human consumption: isopropyl rubbing alcohols do not contain the ethyl alcohol of alcoholic beverages; ethyl rubbing alcohols are based on denatured alcohol, which is a combination of ethyl alcohol and one or more bitter poisons that make the substance toxic.