What is it used for?
If you're like me at all, you've been building something in Creative and need to fill something using World Edit. But, you can't figure out what to put in to use the block you need. With the //blockid command, you can figure out what id you need for any block.
How do you use it?
You use it by holding a block in your inventory. You put the block in your hand and do //blockid. In the chat, it will say (I think in blue font so you don't lose it in chat) which id you use. You could then use the block in World Edit.
Pros:
If you ask someone for an id, and they don't know or don't answer, you can use this command
Easier to figure out the id
Cons:
Could take a while to put into the game
Could not even be used
So, let me know what you think! If you need screenshots, I can upload some later.