Simple, the larger vocabulary, the easier it will be for you to understand the grammar, which will help you learn English quicker as well as make you a better communicator. I know an Italian speaking guy who tries hard at learning the structures of English, but stumbles every time on his short vocabulary, and has to look up every other word in a dictionary. This must be so tedious, can you imagine having to stop every few minutes to find the word you are looking for AARRRGH!
This exercise below will show you how good your vocabulary is, it is made much simpler by giving you a choice of 2 words to choose from. See how you do.