The economics are pretty simple, actually
If consumer spending and business spending have been dramatically reduced, with all the well-known negative consequence on employment and growth, then there's only one place where that shortfall can be made up. Government spending. That's the only vehicle left with which to stimulate the economy back from its moribund state. I know that Republicans can only think of government and government programs as being wholly evil and stupid, but the rest of America realizes that government can at times have a positive role to play. The only real arguments I think are the precise manner of government spending and whether or not $800 - $900 billion will be enough, given that the contraction in the economy could be much larger than that.