Stores the marks online, enabling you to retrieve them when needed, also synchronise between various computers (including if using Firefox under different OSs) or simply to use the marks temporarily from another machine.
It will also store passwords if needed, but I am too cautious to do that.
There is an IE version, which I understand will sync with the Firefox one as well if required. I don't think they have got a version for Opera, but that may just be a matter of time.