I want to understand the rationale behind buying into a short-term Treasury ETF (or mutual fund). In particular, I want to understand how it compares to buying short-term Treasury bonds/bills directly or placing the money in a savings account. For example, I'm thinking of Vanguard's Short-Term Treasury Index ETF (VGSH) or iShares' 1-3 Year Treasury Bond ETF (SHY). Do these ETFs have different different inflation risk and/or interest rate risk profiles than Treasury bonds/bills or savings accounts?

One difference I see is that the nominal return of Treasury bonds/bills is fixed once they're purchased. For a savings account, the principal is fixed, but the interest rate can vary (incrementally). Meanwhile, the both the value and the dividends of those ETFs fluctuate.