There are times when investing in an ETF is more convenient than a mutual fund.
When you invest in a mutual fund, you often have an account directly with the mutual fund company, or you have an account with a mutual fund broker. Mutual funds often have either a front end or back end load, which essentially gives you a penalty for jumping in and out of funds.
ETFs are traded exactly like stocks, so there is inherently no load when buying or selling. If you have a brokerage account and you want to move funds from a stock to a mutual fund, an ETF might be more convenient.
With some accounts, an ETF allows you to invest in a fund that you would not be able to invest in otherwise. For example, you might have a 401k account through your employer. You might want to invest in a Vanguard mutual fund, but Vanguard funds are not available with your 401k. If you have access to a brokerage account inside your 401k, you can invest in the Vanguard fund through the associated ETF.
Another reason that you might choose an ETF over a mutual fund is if you want to try to short the fund.