What is level 0 in Portal 2?

After some googling, I was unable to find any hard evidence of the point or purpose, aside from some other YouTube videos.

As for what its "purpose" is, outside of any Developer confirmation, there's not much that can be said. What I can say is that Devs do often do this, for many reasons. Generally (from what I have heard of similar environments in other games) it's to give them their own testing space, then instead of deleting it, they just remove access to it (again, this particular choice can be for multiple reasons, the main one being "if we remove it the game breaks", or "It's used so that we can store items that will come into play later in the game").

As for why you ended up there, since you were playing on Steam, this can possibly explain. Sometimes Steam Games need the Steam environment to run (for licensing) which can cause processing errors, causing your game to bug out. Either that, or the game itself simply glitched, causing the bug.

Generally, the solution is to load an earlier save, or start again. If the game still runs into the same glitch, especially in the same area, re-install the game, and if it still occurs, contact the sales provider (i.e. Steam) to get a replacement download.