What use is character class? Wouldn’t a class-less game be simpler? To answer this, let’s first understand what character class is.
It isn’t necessarily any category within the game’s shared fiction, which can be best seen in D&D. While Wizard and Cleric might be considered recognized jobs or professions in D&D’s implicit setting, this is much harder with Rogue, Fighter and Barbarian. And in early editions your class might have been Elf or Dwarf. If anything, the actual class of a modern D&D character would be the combination of race and class – or classes, if multiclassing is a thing. I might be an Eladrin Mystic. Likewise, the effective class of a PC from Werewolf: Forsaken might be Rahu of the Iron Masters, with the Lodge of Crossroads, the third item being a prestige class.
A class therefore doesn’t have to prescribe a leveled progression, either. It might make certain choices available, cheaper or simply provide certain benefits when initially chosen. This already makes one of the benefits. You can use classes as a package deal, selling the players certain non-competitive elements that wouldn’t make it on an open point-buy market. For example, in the context of D&D, most players probably wouldn’t pay for a Dwarf’s ability to sense secret passages. But it’s nice to have, even if all you wanted was the Constitution bonus.
The other use of class is a reduction in complexity, not necessarily for the character’s player, even if perhaps a new player might grok the necessary choices to make a D&D character better than Gurps or Hero. No, it’s for your fellow players. Because even if you spend increasing amounts of time on fiddling with your perfect character build, the attention span of your fellow players won’t scale. For example, in a game of Risus, when introducing your character, you can just read aloud your whole character sheet. In D&D that is not feasible, instead you might say that you are Tabaxi Tempest Cleric and convey some useful information to your group.
Although in a game of D&D3.5 even your effective character class might not do, looking at my longest running character:
Brinihera Tiefgang, Dwarven Psion(Shaper)/Constructor/Ardent/Archivist
Which actually undermines the benefits of class in that regard. Note that communication is not only useful, in communicating niches of challenges, a character might fill, but maybe even more importantly communicating how I expect my character to be seen by the world and addressed in play. Like, when I’m a Halfling on Eberron, I expect people to ask me about dinosaurs. I might then tell them that – no! – not all halflings breed dinosaurs. Thank you very much.
So, if your game is more complex than Risus, and if you want to forego the benefits of class, you might want to provide an alternate system for players to succinctly describe what their character is about. In Fate you might cite your High Concept.
Of course, defined classes are especially useful, if you want to depict a particular setting by actually associating the classes with certain elements of that world, be it the orders of Ars Magica, nations and and schools in 7th Sea, or breed / auspice / tribe in Werewolf: Apocalypse.