More reasons:
You have a list but can't possibly afford to register them all. Better someone here get it than the big boys like buydomains or another site that puts them in permanant PPC and likely off the market for a very long time.
You like the thrill of the hunt, but don't have the means to register. However, some NP$ could allow you to buy more. Sort of like commission for finding for others.
You have a domain registered in all but one or two extensions. If all extension are registered, it makes the ones you already have worth more, since a retail buyer could no longer get that alternative for reg fee. If someone here takes that TLD version, you both may benefit, since domains taken in all extensions sell better and their popularity shows value.