Reason?

What is the point or reason behind calculating the Present value of future income or liability as of today?

For example if we have a future income of $100,000 because we have made an investment at a interest rate of 10% for 2 years then its present value would be $82,645 then what does it prove?

Similarly if we have a future liability of $120,000 because we have borrowed money at a interest of 12% for 5 years then its present value would be $68,091 then again what does it prove except that if we pay the loan today then we have to pay smaller amount than latter.

Please answer both of the question. I am really stuck with the reasoning behind them. Please help…