I know this can be confusing because you tend to think of money being worth the face value. So let's eliminate that for the sake of an example that will be easier to understand.
Let's say your friend loaned you rock worth $10 today. He expects you to pay him back an identical rock whenever you can.
Now let's also assume that historically the price of rocks tends to go down every year. At some point you will need to buy a rock to pay your friend back. Because they keep getting cheaper, it costs less to buy the payback rock the longer you wait.
Replace a dollar with a rock in the example and you have your answer. This is known as the time value of money.
In reality, this is priced into the loan (via the interest rate) because the lender very much understands the math going on here. Also, it is more complicated because the longer you delay payment the more interest you pay (pebbles if you will) so it doesn't usually work to your advantage unless they underpriced the loan's interest rate.