In the best of all possible worlds, leaving a job happens on your terms. When you give notice, you ideally do so having already lined up your next job or activity, and with enough money in the bank to tide you over during "funemployment." Being fired or laid off is a different matter, especially if the decision comes out of the blue.
For many Americans, having enough money to cover a $400 emergency expense is difficult. According to a 2016 report from the Federal Reserve, 46% reported "that they either could not pay the expense or would borrow or sell something to do so." Factor in a layoff and the situation becomes direr than even paycheck-to-paycheck living. That's why getting paid severance can be such a boon.