What does the leaf spring do?

The leaf spring is designed to absorb energy and vibrations transmitted to the body.  
Why are leaf spring bushings so important?

Remember that the condition of your vehicle determines how it will serve you. BDS was the first in the industry to offer both internally and externally fluted bushings for improved lubrication flow. All bushings will wear out over time, but BDS bushings are designed to handle some serious abuse.

Specs:

    Grooved construction
    Uniform grease distribution
    Long lasting durability
    Quiet operation

In the box:

    (16) Bushing