What's the reason? For the darkness, I mean...

[Apjjm]

But i am pretty sure the monster can't see you. It just walks away and vanish.

If you run right up to it, it will certainly see you. And kill you.

This i can confirm:
http://www.youtube.com/watch?v=T326-xWrqqE&t=245

The discussion about the darkness is most likely just a device to draw your attention to it. This is probably a balance of scaring with the darkness and reminding the player to light stuff up with the speech - similar to the note in storage. An in-game explanation as to why the room needs to be kept that dark doesn't really jump out straight away - perhaps it was a rather busy area for Alexander's servants, which tended to keep out of view of Daniel.