There are always going to be those for whom these things appear to work, but the laws of probability and the vagaries of Discovery ownership mean that you'll never get any scientific basis for the claims made by the sellers.
In nearly 50 years of motoring, I've seen these come and go, fuel pressure regulators, magnets on the fuel pipes, spark enhancers, all sorts of tins of stuff to add to your oil or your fuel and so on.
In 99% of cases, at best they are simple devices that do no harm, at worst they are snake oil and can safely be ignored.
It's your own money and your own vehicle, so buy what you will and let's hope that nothing untoward happens, but please don't expect me to believe the claims made by the sellers, and don't come back here complaining if you subsequently have problems.
Peter