I'll attempt to keep this answer opinion free!
Your first stop should be the EE3 changelog, and also catching up on the EllisLab blog over the past 6 months. But you'll also need to look at the EE2 changelog to see all the changes since your version that are also included in EE3.
The main changes are the infrastructure and Control Panel but it's where the v3 platform can take you into the future as it's far better structured for expansion, flexibility and new features.
Your websites are already running an old version from May 2012 - this should be considered for an upgrade anyway to the latest v2.x version, if only for the security updates! If you don't appreciate the need for updating regularly, then best read up about Drupal and the millions of websites with possible compromises from attacks in Oct 2014.
But ultimately the answer comes down to what your websites (or clients) want to do, if you/they are happy with the websites and don't need to change as they have all the functionality they need for the future, then there is no point. if you/they want to continue to expand the sites in the future, then you will need to factor in an upgrade at some point in the future as more and more add-on developers have stopped releasing updates for EE2 add-ons to focus on EE3.
There are still EE1 websites in the wild, because that version still suits the requirements of the owners. So there is no essential requirement to upgrade to EE3 unless you outgrow EE2.