In Man of Steel, we start out with a look at Krypton, the birth planet of Kal-El (a.k.a Superman). His father, Jor-El, is the planet's top scientist, and is concerned that Krypton is going to be immanently destroyed. However, the Kryptonian Council rejects his idea of evacuating the planet. This is where General Zod comes in, and shoots one of the Council members.
Now we can sense that Zod had previously been in trouble with Council before, and already know that there was some tension between him and Jor-El.
Reason 1: Development of Zod
A Man of Steel prequel would allow us to explore more of General Zod's past with Krypton in-depth, while also bringing us closer Krypton than ever before. It also would give us the opportunity to explore the reason why Krypton decided to stop giving natural births, and abandon it's colonies!
Reason 2: Introduction of Braniac
More importantly, if you've ever read the Superman comics, or watched the Superman/Justice League Animated Series, you should know who Brainiac is. In Man of Steel, Brainiac is introduced in the form of a hovering robotic being that assists Jor-El, and also defends the Kryptonian Scout ship that Superman finds during his travels.
The reason why we NEED a Man of Steel Prequel is to introduce Brainiac as the Digital Conqueror of the Universe, not the helpful piece of AI in this film. During Krypton's final hours, Brainiac knows, thanks to Jor-el, that the planet is about to be destroyed. So he plots a way to escape the devastation, for Krypton's Legacy to live on. Not only does he plot his escape, he also shrinks and steals the capital city of Krypton, Kandor, and stores it in a bottle. Which leads me to....
Reason 3: Purpose for Braniac
Like any other AI, Brainiac needed a purpose. With the destruction of Krypton, he had no place to protect, or assist. So he searched the Universe in search of knowledge, to ultimately become the Brainiac we all know.
So with a Man of Steel Prequel, we would learn more about the famous Krypton's past, while also introducing one of DC's most infamous super villains! Bringing unlimited possibilities to the DC Cinematic Universe, such as a Darkseid/Brainiac team up, like in the animated Justice League.
So what do you think? Should we get to see more of the crimson planet?