Well..... Yes... they do! Maybe your bike has an practical range of gears for you for the hills, type of riding you do. my trike needed lower gears to get up one hill we have here. there is a limit to how small a gear one can install on the crankset.I also felt I could go faster on the flats than the original gearing allowed. I could have bought a larger gear for the crank for that also. But now the deraileur on the rear needs to be longer to acommodate the increased range. It made more sense to install an intermediate drive!
Ed Service