fruux makes sure that your contacts, calendars and tasks are always up to date on all of your devices. To achieve that it synchronizes all changes. By using fruux you also always have a backup of your data in the cloud. It's simple :)
If you are a bit more technical minded, that explanation didn't really tell you all that much, so here's some more detail:
fruux is built on top of sabre/dav, our open core (if you are interested, also check out our other open source projects). sabre/dav is a great piece of software that enables applications to provide sync functionality via the open standards WebDAV, CardDAV and CalDAV. Besides us, developers and companies around the globe use our sabre/dav technology to deliver sync functionality to millions of users. So even if you don't end up using fruux, but instead some other product which offers sync functionality via the WebDAV, CardDAV or CalDAV protocols, it's quite likely that you are using our technology.