Welcome to Flyertalk.
You have been pointed to the main thread on the Amex Vouchers. You have also had pointed out the two main use case for getting value...
1) Last minute flexible short haul tickets that would have been prohibitively expensive otherwise
2) Long haul redemptions in premium cabins.
There is an awful lot more detail that is covered all over the site and deep in the weeds of those other threads.
Many of us are now deep into this game and find it fun trying to maximise the value from these sorts of offers. It certainly isn't something that just offers great benefits in a passive manner, it's definitely not a 'free flight'.

In order to make use of the Amex or Barclaycard vouchers you need to make an Avios reward booking. That means a) You have to have enough Avios to make the booking and b) You have to be able to find the reward flight availability for the flights you want to take. Neither of these are a trivial matter.
There are lots of tips and tricks for gaining Avios and if you are not getting shed loads of them organically from flying it takes a bit of planning. The main one is churning credit card bonuses which means cancelled your Amex cards and getting a Gold\Plat every two years when the bonus kicks in, cancelling it then wait another two years. And indipendantly doing the same with the Premium BA Amex. You need to completely cancel them and wait 24 months before you are eligable for the Avios bonus again but you can spin each one indipendantly so get two sets of bonuses every two years. If there are two of you, you can each do this. On top of this you will get some Avios for your spend on the cards, but that probably won't be enough to do anything interestng, so you should use the 3x 'boost' feature in your executive club to buy Avios at a cheap rate (you can get up to 300k Avios a year this way at the 'good' rate before it drops to the same rate as normal buying of Avios, which you have already discovered is fairly poor value). Another slightly more effiicient way is to get an Avios subscription. The caveat with acquiring Avios is that you need to have a plan for what you want to do with your Avios before starting to collect them, otherwise you risk being disappointed when you have invested a lot in collecting them, but can't do what you want. So some research and planning will be required.
The other issue is reward availability. For in-demand rewards you may have to plan to ring up at midnight 355 days in advance to secure seats. Others are still reasonably easy to acheive (like US flights) but you will usually still need to plan well ahead. The Amex premium vouchers actually give enhanced reward availability in business cabins as you can book into commercial I class spaces as well as the normal reward spaces. That is a key benefit for many. The other tricks with availability come with having status in the executive club of Gold or higher, so many people go out of their way to achieve status to get these perks. But perhaps discussion of that is for when you have become more addicted to the passtime of frequent flyer reward redemption