Either the scene must be close to eye level, which makes switching inconvenient, or view blocks need to be used which operators may need to reach around, or which occlude the trains during operation. Failure to consider these viewing angles at the onset may lead to the easy discovery of the illusion. The model world will not look as big as intended, or worse, it will look completely unrealistic. Of course, the perennial tension between operation and scenery is not the only reason why we consider the scene early on in the design process. Another reason is expense.
Scenery can be quite expensive both in terms of money and time.
Forests are perhaps the canonical example of this phenomenon. Particularly with the smaller trees that are common on our layouts, it takes an incredible number of trees to make a forest. For a pine forest typical of the Pacific Northwest, for example, there will easily be 100 trees crowded into a square foot of real estate. Even the fastest and cheapest imaginable methods result in a forest that is in the order of 25 dollars and three hours per square foot. Naturally, even more costly lessons greet us when we plan vast expanses of city-scape.