It wasn't a blank space on an old character sheet?
Seriously, though, a role is just a vague grouping of classes that can make some of the same vital contributions to party success as eachother. A Warlord or Shaman can substitute for a Cleric, and your party won't face a TPK for lack or healing. A Warden or Paladin can stand in for a Fighter, and your rear line probably won't be over-run an killed. That kinda thing.
It's /very/ handy for pulling parties together, especially in pick-up games.
It's also nice for pulling a party together when players have specific concepts they don't want to compromise on. If two players both want to be 'mighty warriors,' for instance, they don't have to fight over the 'tank' role, because there are also martial striker and leader options.
The only problem I saw with roles was the execution (or perhaps conception) of the Controller role. The problem with the execution was that all the support or the role was built into exceptionally-potent class powers, rather than into features. The problem with the concept may well have been that that was the point (to grandfather in something like the wizard's - and Cleric's, now I think of it, as a 'secondary controller' - old very-potent spell list).