Roles are useful when building a party. Just think of all those movies with the dramatic phrase "I've been forming a team"
You need someone to take the bulk of melee
Someone to heal
Someone to do crowd control
Someone to hit hard at one foe
And someone to have the 'skills' side covered
Traditionally, the roles have been
Fighter (bulk of melee, the 'tank', somewhat hit hard at one foe)
The Cleric (Healer, crowd control [turn undead anyone?])
the Wizard (Crowd control, hit hard at one foe, sometimes 'skills' side)
The thief/rogue/burglar (Hit hard at one foe, skills)
That being said, they have helped get groups together or give me character ideas based on what is needed, with the OODLES of classes we now have in D&D, the roles are (IMO) fairly pointless, of course, that is saying 4e is 'now'
Sometimes I have, however, done the same character to build to suit as many roles as possible.
For instance
Human Fighter, focuses on archery.
Able to fill the role of 'controller' and 'striker' not to mention hold his own in melee, granted he doesn't enjoy it as much.
That was 3e
In AD&D I was able to pull of the same character, human fighter, focuses on archery, ect. ect.
Still did what he had to do
Even with the White Box I could do this.
I could mix wizard and fighter to make, thats right, and arcane archer (IF he was an elf instead) but I quickly figured that roles are less about combat, more about surviving the dungeon itself.
A spell like fireball can kill a lot of kobolds, as can my bow, but my bow cannot grant me the ability to use scrolls or wands. I can strike hard like a sneak attack, but never can I open doors and disable traps. The roles are not just about combat, but character design.
You need the hard as nails merc fighter, the sly and devious thief, the wise and preachy cleric and the intelligent know-it-all wizard to get through a good dungeon, and a good dungeon is just as much thinking as it is fighting