What purpose does it serve for non-money market mutual funds to hold money market instruments?

The purpose of holding money market instruments for non-money market mutual funds is to provide liquidity for the fund. If the portfolio manager has an immediate need for cash, such as to pay expenses or meet redemption requests, money market instruments are relatively easy to liquidate because they have short maturities and low credit risk. Money market instruments do not primarily generate investment income that provides investors with preferential tax treatment, as interest income from money market instruments is fully taxable at the investor’s marginal tax rate. Money market instruments are not purchased by non-money market funds to satisfy the regulatory requirement of fund diversification, as there is no such requirement for mutual funds. Money market instruments do not ensure that the fair market value of a mutual fund will not drop below a minimal market value, as money market instruments can also fluctuate in value depending on interest rate changes and supply and demand factors. References: Money Market Instruments