True a few years ago when oil and product prices were low, quite a few ships would anchor off shore loaded awaiting someone to buy cargo and prices to climb. ie Southwold. But prices are good at the moment so don't think this would go on as much as it did a few years ago.
Most tankers anchor off shore because there refinery hasn't a berth free or the refinery hasn't room for there cargo, ie Fawley ships anchor at Nab or smaller ones at St Helens simply for reason there isn't a berth available, the refinery hasnt room for there cargo or prehaps there outbound cargo hasn't been produced. In the case of the BW Edelweiss because of its size i would guess its available berths are limited and it is even possible that it may conduct Ship to Ship operations.