Using scale models of proposed designs is useful is some professions. Architecture is perhaps the most obvious one. The average person will not get a very good impression of a building from a blueprint, but decent scale models shows a client what the designer has in mind. This extends to some other industries as well.
Museums use scale models for some displays because the historical items are either delicate, too valuable, or simply not in their original condition. Realistic model trains can show what an historical item looked like in its original context. Model decals provide the smaller details for these displays.
Accurate reproductions of factories and power stations have been used to help with refining operational proceedings. Factory management use these when designing assembly line operation, delivery routes, helping with efficiency consideration or simply outlining safety procedures. More than a few factories and companies keep a scale model of their facilities in the front foyer. It gives clients a fairly accurate impression of the company. Decal paper allows realistic labelling of the various parts of a factory model.
Aviation industries have used aircraft scale models since before there were working planes, the Wright brothers using a wind tunnel and models for research when designing their first flyer. More highly detailed plane models with decals have been used for training, so pilots can identify other types of aircraft.
A more prominent use for models is in the film industry. With few exceptions these put a great emphasis on realism and believability. Thin decal paper provides the realistic details for these models.
Computer simulation does impact the model industry; we can produce a 3D computer image of an object instead of building a model. But computer aided design, 3D printing devices and inkjet decal paper also make model building easier. Waterslide decal paper will continue to have its uses.