Why do they make mockups?
From a more general perspective, mockups serve pretty self-evident purposes. That's not a slight on your question, BTW, its a "don't overthink it" kind of thing; you no doubt use mockups in your daily life. It really
isn't rocket science.
Operations like handling and logistics benefit bigly from mockups. You can make sure your lifting fixture or your truck or the roads you're driving down are all going to work when it comes time to do the thing for real. You can do process verifications to make sure you have right right tools and access--it sounds dumb, but imagine if the person-lifts in the images [in this thread] weren't tall enough to do the job? Or if they were too heavy and sunk into the ground?
You can do fit-checks and operational checks with flight or flight-like gizmos and gadgets, so you can test out subsystems and partial systems in a more flight representative environment. Sometimes, depending on the fidelity of the mockup, you can do structural and even thermal tests to validate or at least semi-validate your thing.
Mockups basically come down to risk reduction--specifically, identifying and resolving high-risk items off of the critical path. You spend a little money up front but you save the otherwise potential (= inevitable) time, money, and schedule impacts down the road.
Fun, somewhat in context story: One time we had to let air out of the tires on a spacecraft shipping container just to get it through the door of a new-to-us facility...
