My company got audited by the IRS. We had a pretty good year in 2014, and decided to save some cash for a rainy day, to not lay people off if things got slow. The IRS doesn't like companies to save money for some reason; all the income taxes had been paid, but that's not always good enough.
Good news, we passed. We'll have to pay the CPAs, but that's not enormous.
The arrival of purchase orders has a Poisson distribution. A quiet spell could be bad news, so it makes sense to accumulate, say, six or eight months of reserve.