One of the regular features we do on Twitter is "Why we need hyphens": phrases that have different meanings depending on whether there's a hyphen. These usually occur when a noun has a compound modifier, that is, a modifier that is made up of more than one word.
The classic example of this is "small-business owner" vs. "small business owner." Is the owner of a business diminutive, or is the business itself small? Depends on the hyphen.
Some other examples of why we need hyphens: A heavy-equipment operator
Because a heavy-equipment operator is not the same as a heavy equipment operator.
Because hazardous-materials training is not the same as hazardous materials training.
Because an invasive-species eradication is not the same as an invasive species eradication.
Because 10-year-old trees are not the same as 10 year-old trees.
Because an anti-child-abuse program is not the same as an anti-child abuse program.
And our favorite: Because 300-odd editors are not the same as 300 odd editors. (Well, not necessarily.)
Why is this even a problem? Part of the reason is that in English, words can shift parts of speech without any changes. For example, a noun can be used as an adjective and there's no special ending or change to mark it as an adjective, like "business" in "business owner." Then an adjective can modify just the noun-turned-adjective or both nouns. So we can pile up nouns and adjectives and other parts of speech in front of one real noun being modified. Hyphens help us direct traffic amid the pileup: they connect the elements of the compound modifier to show that the modifier should be read as one unit.
Now, this doesn't mean that every compound modifier needs a hyphen — they're needed only when a phrase could be read more than one way. A string of adjectives in front of a single noun is fine, as in "a big old white house." An adverb plus an adjective is fine, because an adverb can modify an adjective, but not a noun, so there's no confusion: "an oddly shaped room" can be read only one way.
Reasonable people can disagree about exactly when to put in a hyphen: there are no hard and fast rules, and it's a fine line between clarity and clutter.
Here's a good rule of thumb, though: If the two modifying words are different parts of speech, they probably need a hyphen. Stinky-cheese vendor (adjective-noun) Mixed-up world (participle-preposition) Frost-free refrigerator (noun-adjective) Sit-in protest (verb-preposition) Grass-fed cattle (noun-participle)
Hyphens are signals to the reader. Without them, there can be a chance for confusion. And even if one can make the argument that it's clear what the phrase "really means," any potential stumbling block for readers should be fixed — a smooth ride for readers means they focus on the message.