I've never heard it said that we read merely to increase our vocabulary. That may be a happy side effect, but it's certainly not a fundamental justification.

I think most people read 1) to educate themselves and 2) for pleasure.

On learning new words: my wife and I keep a set of flash cards on which we write words we don't know. We don't always look up the definition immediately, but eventually we do and write it down and then we review the words later until we can remember it's definition.