Why does Warren Buffet suggest to earn money when you need it least?

Because it is about your self-respect. It sets are limit to the extent you can go to earn money. For example, you are already making a decent income from your job, but hey! jobs are not permanent. So you start creating a side income. 

It decides how far you can go to generate side income, without damaging your self-respect, until it becomes your main income. 

You cannot decide this self-respect bar/extent when you are jobless. It will feel wrong. You will question yourself at every step: would I do that if I was already making money? Or am I doing it just under the pressure of making money? So the saying of Warren Buffet that to earn money when you need it at least is justified.