If you collect 1,200 coins, you will watch a scene where Mario saves the princess.
Switch Between Mario & Luigi
Every time you collect 100 coins, your character will switch out. When you collect your first 100 coins, you will play as Luigi. The next time you collect 100 coins, you will play as Mario. This will repeat, with the next 100 coins letting you play as Luigi again, then Mario, and so forth.