The idea is supposed to be to stimulate economic activity. That is supposed to happen if a dollar of stimulus results in more than a dollar of increased economic activity. The following graph from an article by economist James K. Galbraith indicates that some actions are stimulative and some are depressive:
It turns out that some stimilus is actually stimulus anti-matter. Some amounts to using an accelerator, some is close to coasting (no effect) and some is like stepping on a brake.
Do you think any of this information gets a lot of consideration in Congress? Apparently not when many spend their time fighting over what income tax rates should be carried forward. With so many feet flooring the brake, pressure on the accelerator is likely to be overcome.