Metal springs for coil rear shocks – as opposed to air-sprung suspension units – are replaceable in order to upgrade (e.g. replace a steel spring with a lighter titanium spring) or to fine-tune your rear suspension performance. Changing your rear shock spring enables your suspension to tuned for factors including your own weight, your chosen riding discipline and the suspension design of your bike.
Rear shocks can transform your ride from a springy and bouncy affair to a well-controlled experience where you gain confidence and boost your skills. Full suspension bikes use both the suspension linkage and the rear shock to provide control and traction in rough conditions, like through rocky and rooty trails.
Rear shocks come as air sprung or coil sprung. This refers to the spring used. Coil shocks are often more supple, but are heavier, as well as not being as versatile. Ait shock may not have quite the same suppleness, but are easier suited to a greater variety of riders, as the air spring can cater to different rider weights, where you would need to buy a new spring for different rider weights on a coil. Make sure you get the exact right size for your bike too.