Crafting tables are made for crafting things. Right click them to open a GUI to craft something. The crafting GUI is a 3 by 3, giving you 9 spaces, which is more than the 2 by 2, 4 spaced space in the inventory.
At versions 1.9 and 1.9.1, the tables are buggy, but this is fixed in the snapshots/Beta 2.0. :D
Used for crafting most things. Right click to open crafting GUI. I guess could also be used as a decoration in a way. The IDT one is for crafting IDT stuff only, in case if another mod is taking over a crafting.
Bug; Beta 1.9 - Beta 1.9.1: Crafting issues. There's always crafting issues. But mod crafting tables could only do mod and vanilla crafting. It could not do any crafting from any mod.
Trivia[ | ]
In the Beta 1.9 versions, the mod crafing tables could only do recipes from this mod and vanilla.
This was fixed in the snapshot 15w11a/Beta 2.0, where all the tables could do any crafting recipe. (From any mod and vanilla)