First, to collect models that you want to build later. They do go out of production, ones already out of production get harder to find and more expensive as time goes by.
If you say develop an interest in F-4 Phantoms, there're good kits out there now that may very well not be in a couple of years.
The only negative I see is that sometimes buying things is an adrenaline rush, and that gets to be an expensive hobby.