Winston Churchill remarked that hyphens were an abomination. He meant stylistically, of course, not literally, and Churchill's eccentricity must be only amusing, not contemptible, because of the importance of the hyphen.
Importance, you say? Indeed, because every adjective modifies the noun following, thus: Turn off the dirty hot water valve. This is a command to turn off a valve that is clearly identified. It's dirty and it's hot, this valve that controls the flow of water.
But this could be the wrong meaning. What I mean is that this valve controls the flow of dirty hot water. Therefore, I must join the adjectives to form a descriptive unit, thus: Turn off the dirty-hot-water valve. Many constructions in English ignore this rule for clarity, insisting that hot air balloon and used car salesman are universally understood and accurate without the hyphen. The hyphen, then, is used for clarity of meaning, and you have to decide that. The hyphen exists for a reason, and its correct use is a sign of English proficiency. Thank you and good night.