Back in the day, when there were no iPads and we did our homework on the backs of shovels using lumps of coal (boy, did teachers hate grading papers then), we once had to do the following assignment: Write a story about something you did yesterday. Then rewrite it, replacing as many "little" words as possible with "bigger" words. This was an exercise in stretching our vocabularies. I can't remember what grade this was, possibly third, or maybe fifth (it couldn't have been fourth, as I had a particularly memorable and colorful--OK, frightening--teacher that year, so I would remember that). Anyway, I distinctly recall hearing another kid reading one of her improved sentences, which came out: "My friend and I purchased each other Christmas gifts." I have remembered this sentence all these years, because of how it grated on my pedantic little ears. What, I silently wondered, was wrong with "bought"? Swapping it out for "purchased" made no useful difference in meaning, and it wrecked the grammar of the sentence besides. But the kid had done the assignment correctly, and so was praised.
Now, I'm not saying this exercise was a terrible idea--it was just an exercise, and even a rather inventive one. But it did convey the idea that the purpose of using "big" words (two syllables are better than one!) was simply to sound more impressive, more educated. I honestly think that this is still what most students believe when they are made to study "vocabulary" for various tests. I once worked on a vocabulary CD-ROM whose selling point was that it spat words randomly at the user, and he or she racked up points by selecting the correct meaning for each one before time ran out. Now, it isn't news that context makes it a whole lot easier to learn new words, and that a richer vocabulary develops most efficiently, and least painfully, through extensive reading. But the question still remains: Apart from getting through all those damned tests, is the point of improving one's vocabulary really just to sound educated? To impress potential employers and mates with your great big verbal display?
A much better reason--which only occurred to me rather recently--is that language is a set of tools. The more words you have at your disposal, the greater the precision of those tools. If you know only the word "buy," you have a hammer, and you have to use it on everything from a rock to a Ming vase. If you know "buy," "purchase," "acquire," "obtain," etc.--and understand the subtle differences they make in different contexts--you have a scalpel, or a laser beam, or a pair of tweezers, or a brush. Words are for finding and expressing meaning in the most accurate way possible. They're for refinement. A large vocabulary gives you not only options, but a sense that there are options. Maybe you can't even think of the right word immediately--but knowing that it, or at least something close to it, exists will make you pause and work on that particular thought or sentence, until it says what you need it to say. The thought, the sentence, and civilization itself are all better for that effort.