If you were used to eating cereal with skim milk for years, you would sure notice if you poured heavy cream on that cereal instead of your routine skim milk. You would find yourself in for quite a rude awakening!
Like every food -- how the milk or cream is produced, what it comes from and any of its further processing all comes together to create unique product traits. These characteristics make each make product taste and feel different from each other even though they all still come from the same raw ingredient.
Olive oil works in this exact same way. Each different grade is produced differently: one grade is the first press, while others are refined or come from a different part of the olive. How the grade is made produces a unique oil, and you can taste and see the difference between each.
Just like in our example about the heavy cream that you wouldn't put in your cereal, you will want to choose the right grade of olive oil for exactly what you are making.