to be honest, I just set them to non-clashing keys as you can change the key binds
I have no idea what side inventory and extra do, haven't ever come across the item that uses them
But I definitely like the pocket crafting table... That sure comes in handy..
EDIT: Just realised you wanted to know what they do
Pocket crafting table, if you have a pocket crafting table in your inventory, but not on the 3x3 to the far right of your inventory (I'll make sense once you use it the first time) then if you press the key, it brings up a crafting grid using the 3x3 slots on the right hand of your inventory as a crafting grid. I personally use this constantly as it has a nifty little function of balance (Another keybind), where by you set out say, a chest pattern of wood planks, only put down one in every slot, but a stack in the corner, it will balance it out between all the slots, and thus you can craft 8 chests without the hassle of distributing items manually
Kinda mute point after the 1.5.1 update of vanilla, but I fell in love with the function.