It's an additional layer of protection, regardless. All these other games like Runescape and Maplestory use it, so if it were completely useless I don't imagine they would continue using it. It must have some positive effect.

It's not just a 4 digit code we'd input into a textbox. If you look at how it's used in the games I mentioned, it displays the numbers on a grid, and after selecting a digit it randomises the position of the numbers. I'm no techpert but I'd guess this would make it somewhat resistant to brute force hacking.