A couple reasons I can think of:
- You can place orders for fractional shares of a mutual fund. With an ETF you can't, so you'll probably have uninvested cash.
- If you're uncomfortable entering orders with specific prices, or you're worried about bid-ask spread, you can enter a mutual fund order and you know it will be executed at the end of the day at the net asset value (NAV).
Plus, with Vanguard, once you have enough invested in a fund to convert to Admiral Shares, it's equivalent to the ETF (particularly, expense ratio) with less complexity.