Why is $100 worth more today than the same $100 worth three years from now?

Could you elaborate on why the purchasing power of $100 today is greater than what it would be in three years' time? Is it due to inflation, which gradually erodes the value of currency over time? Or is it perhaps related to the potential for economic growth and increased productivity, which could lead to a rise in prices and a corresponding decrease in the real value of $100? How might investors account for this phenomenon when making long-term financial decisions?