The idea behind the webapp is to fix the issue of post-event sharing. Events like weddings, birthdays and other parties usually end up getting catalogued online first, with people who want those images needing to download them one at a time. This way it’s all fairly instant.
The service does require you to log into Flickr to use it, but it uses Flickr’s own authorisation to grant album access to the webapp. You never enter a password while there.