Prior to Disney's multibillion dollar acquisition of the Star Wars franchise, the Expanded Universe took readers, gamers and viewers back to the origins of the Jedi and some centuries beyond the events of Return of the Jedi. Sadly, the House of Mouse opted to consign these numerous tales to the realms of non-canon and much of the fanbase were devastated to learn their favourite EU characters would forever be 'Legends'.
This doesn't mean the stories can't still be enjoyed, but with so much material lost to the realms of non-canon, it does mean that an awful lot of characters lost a lot of the development so many authors, game devs and other media producers had taken the time to flesh out.
Disney were never going to spend such an incredible sum of money on the Star Wars franchise without introducing new material, however, and much of the new iteration of the Expanded Universe utilises existing characters and puts a whole new spin on their backstories and ongoing adventures.
Some characters have lost a bit of their quality as part of the new Expanded Universe, with the likes of Saw Gerrera appearing as a husk of a man in Rogue One, but there are many characters who have benefited immensely from the new EU tales. For some, it may have even saved them.