Grease between leaf springs makes them work better. There have been other methods tried like plastic liners and pads. The friction from rust makes the springs squeak and not yield to small bumps. The ends of the leaves move along the leaf above with each flex so friction is highest there, and they become friction dampers. The ride is much better with grease.

There have been lots of funky tools to get the job done over the years. This is a neat little tool for the purpose. You hammer it in to spread the leaves and turn the handle to inject the grease.