I watch Buy it Now items for a few reasons.

Sometimes sellers have sales and may reduce the price for a limited time if it looks like the item may not sell.

There may be something I'm interested in bidding for but it would be silly to bid for more than a Buy it Now price so I may keep one as a price guide.  On more than one occasion I've found the BIN price has turned out to be the real bargain compared to auction prices so have ended up buying the BIN item instead of continuing to look for something cheaper.

I've got a few 'not in any hurry to buy' purchases in mind where the seller has lots for sale.  I keep them on my list so I don't forget.