If a fuel tank filler neck needs to be replaced on your vehicle, the most likely reason for this is because the original one has rusted. This could be due to age, or having a vehicle near the ocean, or a combination of both of those. It's a fairly durable part that doesn't need replacing very often. A difference scenario for replacing this part is if something happens to the upper end when the gas cap connects to it. If that end becomes damaged or dented in any way, and the gas cap no longer fits or latches properly, you may need to replace the whole thing. Yet a third scenario for replacing this part is if the vehicle has been in a collision. A collision would definitely tweak it out of alignment and it would need to be replaced. This means it's likely the gas tank and attaching hardware would need to be replaced as well. The filler neck the tube, usually made of steel, which directs the fuel from the nozzle at the pump into the tank. The end of the filler neck needs to have a special shape to accommodate the gas cap as well as the shroud on most gas pump nozzles.
From the beginning of the internal combustion engine, there has always been a need to have a tank to store the fuel on the vehicle. At first, the tank was more or less exposed and had a hole and a cap directly on the tank. The fuel would then be poured directly into the tank. There was no filler neck required. As the design of vehicle included more body work for aerodynamics or just aesthetic appeal, the gas tank was relocated under the vehicle somewhere. This made the fuel filler neck a necessity to deliver the fuel into the tank from the outside of the one of the body panels. Fun fact: ever wonder what happens if you pour sugar into someone's gas tank?
As you can imagine, with the fuel tank being located under the car, changing out the fuel filler neck isn't a simple process. You'll need the car on a lift, so taking it the local mechanic is the best idea. If you prefer to perform the repairs yourself, there's some brief instructions to follow.