It makes me laugh it sounds so silly.
I want to buy that Apple computer?
Do you own an Apple computer?
No.
Sorry I can only sell you an Apple product if you already own one.
While Patek is probably among the worst offenders in that regard, a very reasonable consideration is that the stratospherically high-end watches are supply-constrained (partly due to there being so few people in the world capable of making them–I'm not taking about your $5K Rolex or even your $25K Patek, but the most complicated and finely-finished ones that can take a watchmaker months to years to produce just one example), and it's reasonable to sell them to known individuals who aren't just buying to immediately flip them at auction.
It also makes sense to reward those most loyal to the brand rather than allow something to be bought by the nouveau riche who will quickly move on to the next flavor of the month. That's how you alienate lifelong customers.
Or so I've been told, anyway. For all I know it's an urban legend.