One reason I put things on my watch list is so that I remember to contact the seller to ask her to author an 'All About' page for me when I get a minute to think what I want on it.
Other than that, it's usually because something catches my eye while I'm working so I put it there to have a proper look at when I have the time. If I checked the item out when I saw it I'd get distracted and never get any work done.