Why did Greg's dad suggest he open a bank account in Lunch Money?

Greg's father suggests that he open a bank account so that his savings will earn interest. In the book, we learn that Greg's father discovers twelve five dollar bills hidden among the pages of a reference book in the family room. After finding the money, Greg's father suggests going to the bank to open a savings account. Initially, Greg is suspicious. He wants to know whether he can have access to his money whenever he wants.
His father reassures him that he can. However, he also tells Greg that, if he's in the habit of consistently taking the money out of his account, he won't earn much interest. Greg's father goes on to point out that, at five percent interest, Greg could add five dollars to a hundred dollars at the end of the year. Upon hearing this, Greg isn't exactly pleased. He points out that five dollars isn't a lot of money. After all, he can make more money in one weekend than the bank can pay him in a year. Greg's father agrees but also points out that, if the house burned down, his savings would be lost forever. This got Greg's attention, and in the end, he put three thousand two hundred dollars of his savings in the bank.