This is the first thread I have seen that asks the question, Why does the Sandbox exist? Other threads talk about how can one beat the sandbox, what does the sandbox affect, what is the sandbox doing, etc?
Some believe it exists just to ensure that new sites use Google AdWords and thus pay Google to rank well. But Google's Do No Evil motto would then be completely sacrificed.
Others feel it is a technical issue, where Google simply has no more room in the main index and they need to create brand new indexes, which takes longer.
One of the most popular beliefs is that this is to combat link spam, by placing all sites in a holding cell until X months, an intense link building campaign for new sites can be fruitless for a long time.
Interesting thread if you can weed through some of the poor posts that go a bit off topic. These theories are then argued and new ones arise.