What is level 0 in Portal 2?

Is level 0 a placeholder level for when the game doesn't know what level to load? Do any levels that don't have a map assigned to them load this map?

If this is a placeholder, what is the point? Might a error message be better, or simply send the player to the main menu? If this level is supposed to only be accessed by developers, why was I loaded into this level?

Here is my video of level 0. In the description of this video is my story of what happened, which I will copy and paste; No clue how this happened. I was playing steam shop games, and it had said the server crashed, and I clicked "rejoin." I showed up at this level. When you complete the level, the elevator goes up forever. It's so weird.

Edit: I uploaded the save file to my site.

Edit 2: website changed name so updated link

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.