Why did the Saints draft another running back???????

They already have Pierre Thomas, Chris Ivory, & Reggie Bush. Why get a 4th? I like Mark Ingram, but we don't need him. Who will be our starting running back this year? I think Ivory is the Saints best running back. Also, name your top 5 quarterbacks. mine are 1. Peyton Manning 2. Tom Brady 3. Aaron Rodgers 4. Drew Brees 5. Philip Rivers