people who like expensive watches (ie other rich people) can tell that his is expensive, so he's part of the ingroup. it's an impractical amount of money to spend, even at his steep discount, on something that is also functionally outmoded for most people by smartphones, and thus acts as a high-cost (honest) social signal.

some people also like sophisticated physical mechanics (likely including GP, since he can repair watches), but i believe that's typically secondary.

It's kind of interesting that we've come full circle. We started with pocket watches and moved to wristwatches when it was too inconvenient to take something out of your pocket to check the time in the trenches of WWI. Now we've transitioned back to pocket watches in the form of a phone because it's "more convenient."

High end watches serve the same function as jewelry. They're also magnificent pieces of art and engineering in many cases.

If you just want something that keeps time, the $20 Casio is fine. In fact, most people I know just look at their phones. However, there's elegance and sophistication present in a luxury watch that you don't get with a simple digital watch.