What are the justifications for the bailout and stimulus package?

Why do the supporters of the bailout and the economic stimulus package say it is necessary? How exactly is it supposed to help the economy, and why does this justify the cost and long term effects? What are the long term effects? This isn't a rhetorical question, I don't want rants against it. I want someone to actually try to persuade me that this is a good idea. So far, I have only heard of its drawbacks and faults, and can't see how this could possibly help us out of the recession. I want to know why our leaders are willing to spend a trillion dollars on this. Thank you for your time.