Why? Well, in the early stages of civilization, units were rather specialized by culture. Rome did not have Samurai, and feudal Japan did not have Legions. By having these parallel advances (B gives Legions and C gives Samurai) you can simulate early variation in military units. As time goes on there will be fewer such advances (today everyone at the appropriate tech level uses similar aircraft/ships/infantry) but it may add a bit of variety and realism to the early games.

Of course, this is just an addition to the style of the game instead of affecting actual gameplay, but little touches like these really improve the atmosphere for me. The question is whether the AI will choose either advance B or advance C, or will it want both advances. It wasn't uncommon to have Legions and Samurai in CtP1, but that defeats the purpose of simulating cultural diversity among military units.