Money market funds and savings accounts both hold money that is meant to be available and stable. Their legal structure differs completely, and so does what happens under stress.
A deposit is a debt owed by the bank
Money placed in a savings account becomes the bank's property, and the bank owes the depositor an equivalent amount. The funds are lent onward as part of ordinary business.
The depositor has a claim against the institution rather than ownership of any particular asset. The balance does not fluctuate with the value of the bank's loan book.
This is why deposit insurance exists. It protects the claim when the institution cannot honour it, up to a defined limit per depositor.
A fund holds securities on the investor's behalf
A money market fund pools money from many investors and buys short-dated instruments such as treasury bills, commercial paper and repurchase agreements.
The investor owns a share of that portfolio. There is no institution promising a fixed balance, and the value depends on the securities held.
Because the holdings mature within months, the value moves very little in normal conditions. Very little is not the same as never.
The yield is transmitted differently
A fund's yield reflects what its holdings currently earn, so it follows short-term market rates closely and adjusts within weeks of a policy change.
A deposit rate is set by the bank and changes when the bank chooses. The two can diverge substantially during periods when rates are moving.
This is the main reason balances migrate from deposits into funds when rates rise quickly, and drift back when the gap narrows.
Access and settlement are not identical
Savings accounts generally allow same-day transfers within normal banking hours, and the balance is always the full amount.
Fund redemptions settle on a defined schedule, and the price is struck at set times. In most conditions the delay is short, but it is not instantaneous.
Some funds also carry provisions allowing redemptions to be slowed or charged for during severe stress, which is precisely when access would be most wanted.
What each is actually suited to
An emergency reserve benefits from a deposit's certainty and immediate access, since the point of the money is that it is available without conditions.
Larger balances awaiting deployment often sit better in a fund, where the yield tracks the market and the amount exceeds insurance limits anyway.
The choice is between an insured claim on an institution and a diversified holding of short-term securities. Neither is a substitute for the other in every situation.