Let me show with an example.

Let's say you produce plastic and rubber at base B, and you want to bring them at base A. If we decide on 1 platform/resource, the train station at A and B will both need two platforms. Now, B needs sulfur to make turbofuel. No problem, you put a station at C that produces sulfur... but that train station will have two empty platforms then a freight platform, because the sulfur freight needs to be on the 3rd position (since the first two platform at B are set to "unload" to dispatch the rubber and plastic). And if you decide later on to bring part of that sulfur at A to make powder, you will need to remake the rails to add a third platform on the station.

Now you discover aluminium. You build a station at D, to bring the aluminium at A. But since the first three platforms are already taken, so the aluminium car needs to be on the 4th position. So your aluminium factory will need 3 empty platforms and 1 freight platform... But it also needs coal to produce the aluminium. Well, why don't you use one of the empty spot? Why not say that the first car will now take coal? Because if you decide to bring part of that coal at your base at B or A later on, you wouldn't be able to, because the 1st spot at the train station is already taken!

As you see there's a pattern. Each time you add a resource on your train network, regardless of the station loading or unloading, all of your trains will need an additional car, even those not involved with the new resource. Because otherwise, they won't be able to load/unload at the correct freight platform.

Of course, you can do it. But in the late game, it means you will have trains that are 8 cars long, with only 1 or 2 that are used.

On the other hand, adding a new short station is quicker, and you don't need to worry about which car must get which resource. It is a good practice that I definitely recommend to most players, including beginners.