In chess openings players follow memorized moves that have years of theory and analysis behind them. Yet when you get to the middlegame, moves can no longer be memorized because of the infinite possibilities.
This is when strategy comes in to play. Players must use other factors, such as pawn structure and spacial advantages to figure out long term plans and play their next moves. It is very important to understand strategy, as the game's final result depends on which player used strategic thinking the best!
In short, strategies help you limit blunders and help you progress through the game with a good plan.