You don't. They are a way of skipping levels you cannot solve, or returning to a level to retry or replay it, without having to go through all the earlier levels.
There is no Bloxorz 2. There is a sequel game called "On The Edge" which is practically identical to B-cubed. The game called "Bloxorz 2" is a 16-level copy of Briker 1, but does have selectable levels using non-unique passcodes. (see related question)
When you play Bloxorz you will see passcodes that let you skip to that level if you want to play it again to improve your score.
There are two games that are given in searches for "Bloxorz 2." One is a game called "On the Edge" which plays like B-Cubed. The other is a copy of Briker 1 that is called Bloxorz 2. But unlike Briker 1, you can select the levels by 6 or 7 digit passcodes. (see related question)
Select the Main Menu (link at upper left on any stage) Click "Load Stage" and type in the 6-digit passcode. Click "Enter." (selecting a stage with a passcode turns off the cumulative move counter)
You don't need one, but it is 780464.
No, the codes are not a must. They can be helpful if you need to return to a certain level.
No, Bloxorz is a computer game.
No, there are only 33 stages on Bloxorz.
There are 33 stages (levels) on Bloxorz.
the password for bloxorz is 189493
The code for level 16 on Bloxorz is: 000241. The code for level 17 on Bloxorz is: 683596.
The code for level 15 on Bloxorz is: 098598. The code for level 16 on Bloxorz is: 000241.