Any stack of items placed in the empty space in the Crafting Table when crafting a crate will be stored inside it.

If the Crates being used to craft a Large Crate contain any items, the Large Crate will contain them as well.

When broken, both Crates and Large Crates drop wood and the items that were inside. Why would you want to do this? Well, you might just want to have Large Crates full of of coal, I won't judge. But more specifically, Adventure Map makers can put a small stack of items (food items, Lifeshards, whatever currency you're using) in them and then treat them like a pot, crate, barrel, etc in any action RPG. And with that in mind, you can set it in the Config so that it won't drop the slabs.