Some nuance is needed here. Additives are not all without merit,
@WelshGas. No manufacturer designs a motor to be dependent on fuel additives, true (well... there used to be lead in petrol for a reason, once, remember...?). But things change over time.
E.g. when our 2.5 TDI was designed, over 10 years ago, diesel was a completely different diesel from nowadays. These days lots of biodiesel is added, sometimes to up to 10%. And our motor design happens to not like biodiesel. (There is even a sticker on the inside of our tank flap: NO BIODIESEL!) Biodiesel is bad for our type of motor, it is known to clog up the DPF and EGR more rapidly, it can lead to algae in your tank, clogging up the diesel pump, etc. So we try to avoid biodiesel (like with the Ultimate fuel, as described above). But it is not always possible. Therefore our dealer adds a little bottle of Forte every now and then when the engine is serviced to help keep the engine 'clean'.
Our engine was not designed to be dependent on the likes of Forte, no. But the changing fuel world has made it not such a bad idea to use it, in our case. A modern motor, designed for biodiesel use, may be a different story.