The government shuts down, making your grant reimbursement is two months late. A critical piece of equipment breaks down and has to be replaced. An unexpected need for services comes up. These are all examples of when a reserve fund would be quite valuable.
In my time as an executive director, my biggest regret is not setting up a reserve fund at the beginning, and consistently funding it. There was always a need for programs or supplies or more staff, and when we had money, we used it.