Now you may ask why VTI is 0.03% while VTSAX 0.04%. That is because Vanguard of VTSAX provides service, while the service of VTI is provided by your broker.
I don't see what kind of services mutual funds (e.g., VTSAX) offer that equivalent ETFs (e.g., VTI) don't. Wouldn't shareholders call their brokers when purchasing/selling their mutual funds if they don't want to do it themselves online, just like for ETFs? Or do mutual funds offer some other kind of services that ETFs don't?
Note that the example I chose (VTSAX vs. VTI) compare an index-based mutual fund vs. its ETF equivalent, but I am generally in any mutual funds and their ETF equivalents, regardless of whether they are indexed-based or actively managed