( although I have heard that there is a workaround whereby flights, typically charter-flights, will 'fly' to an airport in Southern Türkiye, and then vary their flight-plan to redirect to Ercan airport instead).
It can be used to stretch the range of a fight in case of good weather while having options for worst case weather. For example you can plan Tel Aviv to Antwerp, while selling Tel Aviv to Amsterdam (this is on the edge for maximum range of a 737 with full capacity), then re-plan to Amsterdam if all conditions meet the rules.
There are two parts of fuel planning that make this work. The first is that you need for example 5% of the trip fuel for contingency. That's 5% of the full 8 hour trip on takeoff but that becomes 5% of the 1 hour (past Antwerp to Amsterdam) at the re-planning moment.
The second is diversion fuel to fly to an alternate airport in case you cannot land at the destination. At the departure moment weather for the destination is more uncertain (because it's still 8 hours into the future). That means your chances of diverting are higher and if it's a big storm system you can't be diverting to a very nearby airport (Rotterdam is 10 min flight away and fine for a 737). So you need to plan for holding and a diversion to something an hour away.
Now in flight you get near Antwerp, you're only an hour from Amsterdam (where you really want to go), you have accurate destination weather and you re-plan that one hour. With Rotterdam as an alternate (in case the airport closes for a non-weather related reason). Contingency is 5% of that 1 hour, and typically you will have saved some fuel in the first 7 hours due to shortcuts because it's "plan for the worst" at departure time but reality is more favourable. Now your calculations for the 1 hour extra all meet the rules, so you go there.
Maybe 1% of the cases with very bad weather and exceptional circumstances you may end up in Antwerp instead of Amsterdam. Then you re-fuel there and make the short hop to Amsterdam afterwards. An hour or 2 delay on a bad weather day isn't really unheard of for passengers.
I think I'm missing something – what if somebody actually wants to get off in Amsterdam? Does the ticket come with a little asterisk saying "you're likely to end up in Antwerp"? Or are you talking about chartered or private flights?
I personally would be annoyed if I booked a non-stop that was expected to have s top by default but I guess if it works out a high enough portion of the time it doesn't really matter
You go to Amsterdam 99% of the time, only when the weather is bad you could end up in Antwerp. But that happens to "normal" non-replanned flights as well, if weather at the destination is bad they could also divert to Antwerp.