The same thing to running a business without enough capital to sustain. Because investment doesn't give us instant return we can fully rely on it, maybe months after. That is how important to have a least another source of income to back it up while waiting for our business or investment to grow. We can just quit working on other people if we are really ready and when our investment/business can already support our living. In order to have a good living, we also have to make multiple sources of income which is somewhat hard for poor people and I feel how hard it was.