Vocabulary is important, and I'm not just saying that because some middle school English teacher said so. Vocabulary is about learning words that allow you to talk about a particular subject matter.
This is why just about any field has its own jargon. Yes it helps to shortcut the conversation, letting you get 15 words out of the way and just use the words you need to get down to the real discussion. But vocabulary does more that that.
Secret: Vocabulary allows you to THINK about new concepts. For a LONG time, Mathematics had no concept of "zero". It wasn't that people didn't know when they were without something, but the word "zero" allowed the concept of nothing to be thought of mathematically.
I heard an NPR show, probably "This American Life", where they looked at the difference in a school for the deaf in a developing country where the students developed crude a sign language on their own. These students had no real conversational language before and couldn't read or write in any language. Over the course of years the sign language at the school naturally evolved, rather rapidly. Years later, people had students from multiple "generations" at the school take a series of tests. And there was a cognitive gap between the early students who used the more primitive sign language, and those who used the more advanced later version. These people weren't stupid, but the limitations of their language ... their vocabulary limited the concepts their minds could easily address.
Have you ever read a book, knew you liked it, but really didn't know why? Happens to me all the time. But as I've interviewed authors this past year, I'm just beginning to develop a bit more of a vocabulary about writing and story-telling. In the process, I'm developing a much clearer idea of what I think works in a story, and why, than I did last year this time.
The thing is, they don't have to even be new words per se; but learning new definitions even for old words in a new subject matter makes a difference.
Take the word "low". A "low" baseball pitch means one thing. "Low" when referring to "low fantasy" means something all together different.
So what is my point? Words are important, and the more you understand the vocabulary of a subject matter, the more you will gain an appreciation for the structures inherent to that subject matter. You really do have to learn how to learn.
This is what your teacher, who had you drill a set of twenty words each week for a test on Friday, never told you. Learning vocabularies will increase your enjoyment of what is around you and give you a match to light your lantern so you can see in the otherwise (proverbial) dark forest around you.
Reading, and vocabulary, really are fun-da-mental :)