Games vs. Game Lines
This is taken from the article written by Mike Martinez on RPG.net here.

The Benefits of Structure
Take a look at the most popular role-playing games around. Whether they're number-crunching battle-fests like Dungeons & Dragons or angsty, character-driven talkers like Vampire: The Masquerade, they have a similar structure.

No, I'm not talking about dice, or theme-and-mood or whatever. I'm talking about some of the most basic concepts in role-playing. These games - and nearly every game out there - have character options. Classes, clans, tribes, archetypes, careers, whatever you want to call them. These are the things that people have in mind when they read a game and say, "I want to be a...."

Some games have steadfastly refused to force characters into a given mold - take Call of Cthulhu, for example, or the old RuneQuest. In those games, you just created a person, and then chose "career paths" or whatnot during the course of play. It's a much more organic way of developing player characters, I'll grant you, but not very conducive to marketing supplements.

Let's go back to D&D and Vampire, two of the biggest games around. Of course, D&D has classes. And Wizards of the Coast has smartly put out sourcebooks for many of those classes, such as Sword and Fist and Song and Silence. (Not to mention the dozens of class-based d20 System supplements out there from other publishers.) And then there are race books, magic supplements, etc.

Of course, fans of White Wolf's games scoff at such silliness as class...then go and choose from among 13 clans...or 13 tribes, nine traditions (and five technocratic conventions), and a variety of guilds, kiths, dharmas, houses, factions, creeds and amenti. And Vampire, Werewolf and Mage had not one, but two splatbooks for each clan, tribe and tradition - first/second edition and the revised edition. Wow. Hats off to the marketing folks

The point to all this is that RPG systems are designed to help players flesh out their characters through pre-set stereotypes, whether they be classes, clans, careers or karmic duties. Each of these options, as presented in your average core book, takes up a few pages. Quite naturally, players love more information on their particular character...it helps them develop their characters into well-rounded personalities as the player explores how their individual PC is different or similar to their average colleagues.

And, thus, these are great opportunities for supplements.

I'd never thought about it, but it's true. White Wolf did scoff at "classes" and then created something very similar. It's not the same as the old D&D classes, which really restricted you / penalized you when you multi-classed. In fact, D20 has actually re-tooled their classes to make it more like White Wolf's classes... er, templates, clans, archetypes, whatever....