Define balance. explain the difference between formal balance and structural balance.

Balance is the composition of a piece; the negative and positive spaces that make the piece visually important. 
Formal balance is when everything is symmetrical. Think of a butterfly. When you split him in half, his left wing will math his right wing. 
Structural balance (I think your teacher means informal balance) is when the piece has elements or objects placed in a complex way, so opposite of formal balance. 

Because most artists loved the way that their men or women that they were to paint/sculpture just the way that they had when they sat and posed and they thought that it looked more i guess