"Cash" in balance sheet accounting-speak isn't limited to the dollar bills under Tim Cook's mattress. It refers to a large number of essentially "cash equivalent" things that can be converted to cash quickly and easily, albeit at essentially risk-free rates of return.
You maintain a cash reserve so that, if there is a (hopefully) short-term market correction, you're not stuck with having to sell assets in a dip to meet payroll, etc. Not really much different from people in that regard although obviously at a different scale.
