Companies with large balances park them in securities to earn a yield. These sit next to cash-and-equivalents and are usually included with cash when computing net-debt, because they can be sold in days.
The risk is duration and credit. A portfolio of longer bonds carries mark-to-market losses when rates rise, and those losses can flow through accumulated-other-comprehensive-income rather than earnings, hiding in plain sight.
Example: Northwind Tools holds $75M of marketable securities alongside its $210M of cash, mostly treasury bills maturing inside nine months, earning $3M of interest-income.
Related: cash-and-equivalents, net-debt, interest-income, accumulated-other-comprehensive-income