Herbal constituents can be released into and stored in various solutions such as water, oil, vinegar and alcohol. Some liquids (called menstruums in herbal medicine making) facilitate the release of different compounds and can be more or less effective depending on the plant and it's properties. Below are several different techniques for extracting herbs with alcohol from Ceara Foley's class at the 2016 Herbal Conference.
Generally, alcohol is a better menstruum than water for the complete extraction of plant constituents. Various ratios of water to alcohol will dissolve most all relevant ingredients of an herb while acting as a preservative. Tinctures can also be made with glycerin or vinegar although not with the best medicinal results for most herbs. I would use the menstruums for nutritional herbs or very mild tonics.