Lots of reasons why people watch.
Some are competitors.
Some are looking, but don't have the money (or some other reason) to buy right now (and the link you gave had 4 available, so no rush). So watching used as a reminder.
Some are looking, and are using their watching list to do price (and other) comparisons.
Some use the watching list to show to other family members / friends.
Some are potential competitors seeing whether the item is worth the trouble of selling on eBay.
Lots of different reasons.
