Knowing the definition of a Greek word is certainly important; for you can't interpret a sentence if you don't know what the words mean! Yet there are pitfalls to watch out for when looking words up in a lexicon, or when reading about them in a commentary. Let’s illustrate one such pitfall by using a language you know well: English.
Let’s say you didn’t know English well, but wanted to improve. You heard a stranger on the bus use the word “nice” in conversation, and so you pick up your English dictionary. Before you read the definition, you learn that nice is an adjective, and that it comes from the Latin adjective nescius, which means "ignorant."¹
Does that information help you understand how the guy on the bus used the word “nice”? No, not even a little bit. Yet people do this with biblical Greek all the time!
Listen to the Christian linguist Moisés Silva:
We must accept the obvious fact that the speakers of a language simply know next to nothing about its development; and this certainly was the case with the writers and immediate readers of Scripture two millennia ago. More than likely, even a knowledge of that development is not bound to affect the speaker's daily conversation.... It follows that our real interest is the significance of Greek or Hebrew in the consciousness of the biblical writers....
What you, as an English learner, need to know is how the word “nice” is used today by English speakers, so you can understand people’s conversation with you. And as a student of the Bible, what you need to know is how biblical words were used in Matthew and Paul and John’s day—not in Plato or Homer's day—so you can better understand God’s conversation with you in his Word.