This is the score to the 1812 Overture by Pyotr Tchaikovsky. There's more than one reason this is important to composing. For one, this is a piece synonymous with great classical music. For another, this is a score, which is every part in a given piece put together for the director to read. That is important because it shows the big picture of a piece, as a composer must see it. If not for scores, composers would have a difficult time writing music reliably.