My job gives me quite a bit of exposure to the financials that underly gift vouchers so I'd be happy to answer any questions.
While the cash definitely doesn't sit in a tin under the CEO's desk, there is an accounting entry for the liability.
Saying that, there is a very predictable percentage that aren't redeemed within the validity period. Unclaimed vouchers & gift cards (known as non-redemption) are a definite income stream (recognised at time of sale so it's instant profit) but usually not the reason for the program.
Often vouchers are sold at a discount (groupon, living social etc.) and that discount is only possible because of the non-redemption percentage offsetting some of the discount.
Also, third party costs can be quite high (10-20%). This refers the money paid to supermarkets, post offices, Westfield etc. when they aren't sold direct.
In my experience, gift vouchers/cards would not exist in many mature businesses without non-redemption. Thats the way that companies can justify the hard costs and internal resources that accompany the services. Sure, there will always be some people who realise they missed out on using their voucher however for the most common gift cards 1 year is pretty sufficient. Most people make plenty of trips to the supermarket/shopping centre/hardware store/cinema in that time. And if you don't, maybe you gift giver doesn't know you very well.