Why do businesses hire?

For most people we interact in the economy in similar ways. We work, earn a wage or salary, we consume, save, invest, etc... A big part of this is the availability of a job and what it pays. In the midst of the worse recession in my lifetime, everyone has heard a lot of soundbites on employment, tax policy, and job creation. It seems that way too few folks know much about our economy and how it works.
I will comment here on one now and more later. Most aspects are fairly simple, given our relatively free and free moving system.
Why does a business hire?
Businesses hire when workers or employees are the restraining resource while increasing output. If you can just work harder, longer, smarter or turn the machine up or buy a better one to get more output then lacking workers is not the constraint; don't hire. People are however frequently the constraint.
Now this scenario all assumes the business needs the added output. Today, that is the rub. Today, businesses need customers more than anything.
Now take that fact back to the discussions in the news. How can we (the government) get more demand out there? In these times the most efficient way is to put money in the hands of those that would spend it immediately, these are the poorest and the unemployed among us. Simple.