There's no reason for the average investor to trade ETFs at all when no load mutual funds exist. You can use vanguard directly or etrade (or a billion other companies) for that.
https://investornews.vanguard/lower-expense-ratios-on-21-van...
For non-Vanguard funds, ETFs are generally more tax efficient than mutual funds because they can avoid capital gains distributions in ETFs, but not in mutual funds. Vanguard has a patent on the mutual fund method.
2. Intra-day trading has some value to some people, and
3. As others have pointed out, fees can be lower for the same dollar investment.
ETFs are available from any place that has access to the stock market.