Given I own a $15k road bike, a $10k road bike and a $7.5k road bike plus have previously owned a $5k road bike I guess I can answer your question.
Next to bugger all.
Now, I'm not saying that there are no differences, there are. However, you can get a very good CF race bike for $5k that can win races. Beyond $5k though, the advantages are increasingly marginal.
Take my $15k Pinarello Dogma F, complete with Dura Ace 9200 and Zipp 454 wheels. It's awesome.
But...the difference between it and the F9 with the exact same components is the choice of carbon sheets. The Dogma F uses the top-of-the-range American stuff whereas the F9 a tier down. Real-World difference? Less than 100g. Seriously. Price? $5k cheaper.
Take it down a notch to the F7 with Ultegra and we arrive at $5500. Is it going to be costing a lot more watts? $10k's worth? Nope. Hardly noticeable. You pay for the materials used as much as you do marginal watt saving gains. Are those materials going to be much less durable, less quality? Nope.
So do you 'need' that $15k bike even as a Pro? Well, it depends. At WorldTour level literally every second can count and the level of difference in watt savings are literally down to that at the speeds they can hold.
Why would someone who has to buy one themselves get one then? Simply because we can. It's the Halo model. It is really nice. It just isn't going to make us mere mortals noticeably faster or win us anything.