Sometimes there are people sending "*****bank",etc,
is not it a violation of the rules to have multiple accounts?

No. You are allowed to have more than 1 account, usually people also have their own bank account. FYI you aren't allowed to join in any contest in Bitsler with your other account
does anyone know why they do this?
i have always seen these "tips" from xyz to xyzBank and i have never been able to figure out what could possibly be the reason why anyone would do such a thing! what is the point anyways, just keep in one account or withdraw to your wallet if you want.
To avoid gambling everything away that's why they have an extra account in the site solely for storage. Also by doing this you're saving fees since you accumulate first your profits before making a withdrawal.