I think if it is a fragile and/or pricier, or 'one-off' item, some sellers like to know the item will be handed to the recipient & signed for, rather than left in a porch, or 'safe place', from where it could be stolen - or claimed to be stolen...

Even if eBay cover you for loss or damage, it would be better to have the option of a service which eliminates as much risk as possible, where more valuable items are concerned.