CC is the new "franchise" mode....basically franchise was taken out in favor of CC.
There are alot of similarities though...like trades,free agency the draft and so on. In CC though,you earn experience points by meeting certain goals,and by doing practice each week,then upping whatever player attributes you wish....think of it as sort of a "mini rpg".
As far as the advantage goes for playing online solo,is changes can be made without the need to wait for a patch...but the downside is,if EA's server is down for whatever reason,then you can't play your career.
Luckily I haven't had any issues with online (knock on wood..lol) but if updated rosters are your "thing",then without gold you can't get them (I believe).