A lard press is used to squish the rendered lard out of the cracklings ... if you are butchering your own hogs and rendering your own lard.

They are also useful for squishing fruit for jellies and squishing almost anything that needs to have the juice squoze out of it.

http://collections.museumca.org/?q=collection-item/h99069 pic and explanation

They are expensive because so few of them have survived - they rusted out or were sold for scrap when home lard making was not common. they also had very few other uses, and they are BIG ... if you are getting rid of stuff you don't use, the lard press is right up there with the giant canning kettle.

Here is a link that might be useful: More than you ever needed to know about lard presses