What do people buy 3D models for? I am just wondering why people or companies purchase 3D models in the first place despite their cost.

I have been planning on making 3D models for sale and I have browsed through websites selling them like TurboSquid or CGTrader, but I start to think about how people use 3D they purchased. I mean, the collection of 3D models on these websites are huge and there are hundreds and thousands of amazing artworks, but I just can't figure out why companies buy them. 

For instance, some models are shops like a Starbuck outlet, a skyscraper or industrial buildings. Why people buy them? Are they for rendering? (but what's the purpose of rendering a single building) Are they for product demonstration? (but then it is almost impossible to find the exact building that matches the companies' requirements) Are they for animating? (but a scene usually composed of quite a number of buildings and buying lots of them would be really costly)

This situation confuse me even more when I am looking at expensive, high-end, realistic models. They look stunning but why on earth will someone buy a single high quality 3D model for? (Like a futuristic lazer gun)

I hope someone can answer it specifically on the reasons to purchase 3D models, I have watched some videos regarding this issue but they just give explanations that I just cannot understand (like buying a single cat model for animating :O ) Any help is appreciated.
The main reason is because it's cheaper to buy the final model than it is to pay an artist to create the model. Since the model can be sold any number of times as well, the original creator benefits despite selling it for less than it cost them to make. I've used models that cost over \$200, but would otherwise take well over a week to create, and that's assuming I could find all the references needed to create the model.

As to the types of models and their uses, that depends on the situation entirely. I would imagine a lot of the models like buildings are used as backgrounds for ads or commercials, or as background models for animations. Depending on the situation as well, the models could be used as the focus of the animation, but as you mentioned that is pretty rare as finding exactly what you need can be tough. 

I can (try to) lay out a hypothetical situation to make things more clear. Say a company wants a 30 second ad created involving a flyover of a city and the camera then drops to a parking lot where something happens. Assuming none of this can be filmed in the real world, there's a lot that needs to be created. Let's say that it would take a single artist 2 weeks to create the entire shot from scratch. If the artist gets paid \$200 a day (probably on the low side) and works 6 day weeks, you have a total cost of \$2,400. That's not including any textures or references that may need to be bought. Now lets say there is a 3d city scene available for \$500, plus a parking lot scene for \$200, And a fleet of cars for the lot for \$400. Some background images could be used to speed things up, adding \$50, and some sky HDRi's for \$100. We're now sitting at \$1,250, but there's probably a couple days work for the artist to add it all together and create the final elements or modify the bought ones. Say that adds another \$400. We can round the total cost to be \$1,500, or \$900 less than paying the artist to do everything. This isn't entirely accurate of course, as it's just some random numbers and times, but the point is that despite the costs of the models, which can seem high, time is money, so it's a matter of what is the cheapest.