Back during my high school English classes – or college, so many years ago it’s hard to remember – the grammar books had a chapter on dictionary use. The dictionary is not just a place to look up word meanings.
My Merriam-Webster’s Collegiate Dictionary contains a foreign dictionary, a grammar book, biographical names (history book), geographical names (a word atlas), and other nifty sections, all rolled into one. Granted, most of these are stripped down basic definitional (that is the adjective form of the word definition – I looked it up – but my spell checker does not recognize it) forms. But in a pinch you can get an idea.
Word definitions also include etymologies: the date that words were coined. They also include pronunciation guides, though if you are not into linguistics, this can be not as easy as hearing them online. The meanings for each word are usually ordered in popularity of use. Graphics are included such as the periodical table of elements. And for the writers, proofreader’s marks.
If you are a writer and don’t have a thesaurus, the dictionary contains synonyms to give your writing some spice. But words are different, so subtle differences in meaning can totally change what is being said. So important in poetry, where condensing is pretty much the rule, unless you are writing an epic. And then that rule may still apply.