This is an interesting topic and the sort of thing I like to think about in my spare time!
When I was a kid, I made models very much so that I could obtain the finished, end result. I wasn't going to run out and buy a real Spitfire (I'm still not!) but I could possess that miniature Spitfire in my hand and admire its shape and form.
As time has gone on, this has changed. I still do appreciate the end result, and very much enjoy glancing at my finished pieces from time to time. But the emphasis is much more so now on the process.
I find the act of building to be very relaxing (for the most part), meditative, and it's a wonderful creative outlet. I enjoy working with my hands and making something out of nothing, and modeling supports that very well.
Also, nothing forces you study a subject matter you are really interested in like building a miniature replica of it! You will see and understand aspects of the subject you did not and would not have understood, but for the act of building the model.
And then there is the research. The historical research but also the model making research. Does anyone make a kit of X in a given scale? Which one is the best? What the problems with it? Who sells it at the best price? What paint colors will I need? What's available in aftermarket?
I have often said I enjoy the act of planning for a project more than the actual building. I think it's about even in reality, but I really do enjoy the planning stage.
Lastly, modeling allows me to explore objects which I find aesthetically pleasing. I don't think I could build a model of something I find ugly.
