I got $1300 in vouchers so far this year. The most convenient way to use them is at any airport for in person ticket purchases from an agent.
You can use them for date change fees or baggage fees too. If you buy your flight far enough in advance you can mail in a voucher, though I’d rather just buy the ticket next time I’m at the airport.
The vouchers can be worth more than you think if you can cover the entire cost of a flight with one, because then most of the taxes and fees are waived which are not small. Don’t forget to ask them to do this if they forget.
If your voucher “expires” in a year, a lot of times they will extend it with some fee deduction ($100?) but again you can extend it just by using it for a baggage fee and they’ll reissue the remainder with a fresh year remaining.
Vouchers? Why not cash? Vouchers only ensure that you have to deal with them again.
There's a number of reasons that I haven't flown since 2007.