We learn vocabulary for two basic and important reasons:
1) We would like to be understood
2) we would also like to understand what others are trying to communicate to us.
We don't want to kind-of understand.
We don't want to kind-of explain what we think.
That is how misinterpretations occur; that is how misunderstandings take place. We should be able to discern exactly what is being communicated to us regardless of what form the communication takes place: oral or written. The reverse holds true as well. We want to be able to articulate our thoughts, so that the other person knows exactly what we mean.
Therefore, we can intuitively recognize the specific challenge where vocabulary is concerned: there are times when what we write or say is relatively simple, and there are other times when the thought we wish to communicate is more complex. We might even say that there are times when what we want to communicate is simple, and yet, it is just a tad different from the norm requiring further description and definition. In the following example we can see the meaning change by degrees as the vocabulary becomes more advance: a) dark eyes b) dark, sultry eyes and c) hypnotic eyes that were dark as midnight.
You get the picture. The vocabulary in our example becomes progressively more advanced as the idea being communicated is more exact and detailed.
So, let's revisit the question: Why learn vocabulary? So, that you can be understood exactly, and you can convey your thoughts and ideas, even the complex ones, in clear, concise language that leaves no doubt as to what you meant. In addition, you want to be able to understand exactly what your are reading or hearing, even when it is complex or unusual.
The best way to learn vocabulary? Read. Read. Read. Read every day - even if it is only ten minutes. When you read, have a dictionary close by or use dictionary.com, but don't just skip over the words you don't know. Take the time to look the words up; when you learn new words in context, meaning being used to illustrate or convey an idea, then their meaning, or definition, will tend to be easier to recall the next time you see the word.
Don't be surprised if you hear yourself, or your student(s), using new words within a few weeks of starting ANY type of daily reading program.
Hope this helps, and remember, teach for success and let your students be the star everyday.