because this istheir business, banks collect the savings of the clients to used for loan of the other clients, also when clients avail loan to the bank they incurred interest and some interest was posted to the savings of the savers, for example most of the banks was used 0.005% for the interest of the savings of the clients while if the clients loan to the banks they incurred 3% interest from the clients, thats why if the rotation of the banks were broken they have a posibility that banks will turn into bankcruptcy.