Indeed, as you could redeem for cash instead of a voucher. Said differently, by redeeming for a voucher you lost $20 that could have been redeemed for cash.

Example: car rental is $40 and you have $20 worth of Discover points. You redeem all points for a voucher. Result is, the car is free and your point balance is zero. Alternatively, you could redeem all points for $20 cash and use it to rent the car with another $20 out of pocket.
The difference between the first choice and second choice is the $20 out of pocket, which by definition is the intrinsic savings of the voucher.