Why the adjustable spring tension on the cage of Shimano rear derailleurs? Why do some Shimano rear derailleurs have an adjustable cage spring tension and what is affected by increasing it (default is low tension)?
It's a way of mitigating chain slap and chain retention issues, primarily for more aggressive riding styles. The tradeoff is drivetrain efficiency and shifting smoothness. It's a feature that's basically been supplanted by rear derailer clutches.