For me it's the actual process of building... creating a miniature. It's not limited to model cars for me. In fact, I'd say that in the past several years I've turned to other subjects more and more, so that now actually most of the models I build are not of cars at all.
My recent models include a San Francisco cable car, several old (historically significant) locomotives, several highly-detailed wooden ships (including the Nina, Pinta and Santa Maria, and a Mississippi paddlewheeler), two stagecoaches, a covered wagon ("prairie schooner"), several WWI fighter planes, a few motorcycles, etc.
I still build cars too; model cars are what drew me into the model building hobby in the first place, but lately I'm finding myself drawn more and more to "other" subjects out there. But no matter what I'm building, for me it's the building process that I like.