So the only purpose of the bank is to give players free money? I don't see the point. Banks have purpose in real life: Your money is kept "safe" and the banks have YOUR money to spend. So while you basically give the banks free money, they entice you with the concept of interest. This is useless in Graal though because you giving the bank Gralats doesn't benefit the bank at all(since it's not actually a business).

Besides that, there's no reason to have a bank since there's no real way to lose money in Graal. It made sense way back when you lose gralats when you died, but your gralats are safe and sound sitting in your profile.