I expect there could be several reasons.

I would think the most common reasons are if someone is not sure if they want to buy it or not but if it's on their watch list they will be able to find it easily again, and also for price comparisons of common items.

I will also put small (inexpensive lightweight) BIN items from a seller who also has auction items that I'm bidding on. If it is still there at the end of my auction bidding I might buy it, but that might depend on whether it can be added to basket to include with postage discount, or whether you have to pay immediately with separate postage.

I have one item still on my watch list that I have already bought -  a small car part which turned out to be faulty, or at least did not do what it was supposed to do on the vehicle model/year it was stipulated for.  I had a long delay with other vehicle problems before it was fitted so it was far too late to get a refund, and the garage ordered me another one.  I kept it there to remind me to let the seller know in case his other identical parts have a similar problem for that vehicle model.  I am reminded every time it comes round, but I haven't got round to it so I may as well delete it.  And there are probably another couple of items permanently on sale that I haven't got around to deleting.

I can't see the point of anyone watching items to 'wind up' a seller, or why a seller would be particularly wound up by it.