If you are new to Mob Wars, you probably do not earn enough to do LandBanking (see earlier posts) yet. You have little choice but to pay the bank tax of 10% each time you deposit, just to keep your meager savings from the more professional mobsters.
In your early stages, you will be growing your income doing jobs and paying the banker. Buy Villas and Restaurants when you can. By the time you can afford the Apartment, you should be able to begin your LandBanking.
Banks should then be used to store cash for your Hospital expenses. I have also used banks to store leftover cash, after making all my purchases and land bank deposit at the end of my gaming session (no, I can't play this all day).
Newbie Tip on how to use the Mob Wars Calculator:
Based on the ROI recommendation, you will be prompted to buy Hotels and Restaurants even though you can't afford it yet. Just plug a high number in those fields (like '99') and all will be fine. Remember to remove the number plug once in a while to check if you can afford the Restaurant or Hotel. You can stop using the number plug once you can afford them.
Plug a higher number for the Properties that you do not yet see in your game.