Sorry for the short answer, but it should now from today giving an explanation.
Furled Sails: These are sails that are in a partially lifted position
On a real ship at sea, the sail surface was reduced many times with strong winds. In Harbour many sales were furled too -letting a few set, to maintain the ship's position on the anchor.
In Ships modelling it is mainly the lower sails and sometimes the topgallant sails there are furled. In my point of view, it is mainly done, to give a better view of the deck's details etc. will attach a few images
p.s. both images are from the book: wooden ship modelling for dummies. A preview can be seen on the Net, but if you want to see all- pay