That's the "cheat room". It was originally my controlled combat testing area for companions and equipment and the unreachable platform is still there for me to test bug fixes, but I added a bunch of stuff and allowed access back when the game first launched to make it so people didn't need to save edit for the most common things people would save edit for and potentially break their games in the process if they weren't sure how to do it. Either not wanting to play the game normally, as there's "dev" weapons (no, I didn't use them while testing the game), instant level up items, the companion Pierce well before you could unlock him as well as the ability to unlock a broken companion, and some various other stuff, or for people who want to test companions or equipment in a controlled area where you can't get a "game over" as you can get to the combat dummy and set some options. You can get inside with "Storytime" (no quotes, case sensitive). Keep in mind that equipping either of the weapons or using the other companion (not Pierce, he's fine to take at any point) you can unlock inside will stop you from being able to access the puzzle fights until you unequip them or go back and remove the special companion.