Why do people want more gears int he back when they could have more gears in the front? Well, the rear derailleur is a marvelous piece of engineering that moves the jockey wheels so that they stay pretty much the same distance from the cog the chain is running on and maintains the tension on the chain all of which is optimized for quickly and accurately shifting from cog to cog.
The front derailleur is a crude slot your chain runs through that you use to shove the chain one way or another.
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