Explain what Pausch learned at the grocery store with his mother?

From the text:

Now that I’m an adult, you’ll never catch me with less than $200 in my wallet. I want to be prepared in case I need
it. Sure, I could lose my wallet or it could be stolen. But for a guy making a reasonable living, $200 is an amount worth risking. By contrast, not having cash on hand when you need it is potentially a much bigger problem.