There will be many reasons, but most people long into buying also occasionally sell. So it can be important to buy at the right price. That may still mean a lose, as I tend to achieve on most non-Rolex purchases, but at least the losses aren't more than the enjoyment.
It it would be best to buy something you really like, for the best price and then just forget about residuals - but there is a reason you don't buy Ebel, GP, etc. Etc. At full RRP