I think ultimately people are either going to buy a watch because:
1. they really like how it looks, feels, its coolness factor, the lume, the awesomeness of the movement or its decoration, or for all manner of other subjective reasons; or
2. (whether they really like it or are not 100% sure), they think it's a good investment or low-risk because there will be a ready 2nd-hand market at respectable prices if they do decide to let it go.
I tend to acquire a watch for the former rather than the latter reason.
However, because I'm at a point where my watch collection is pretty big, I do ask myself first if a particular watch I like fills a hole in my collection or is sufficiently different from other watches in my collection.
Thus, when I had some shopping vouchers to spend on a watch, my shortlist consisted of watches with complications hitherto not found in my collection. I did find other watches I really liked, but consciously said no to them because they were too similar to what I already had.
Ditto my recent acquisition of a rectangular vintage Longines. I decided to hold off circular vintage watches, no matter how pretty, because I had too many already.
Of course, when something really grabs you, you'll always find some way to distinguish your new acquisition from those already in your collection