Why do investors invest in index funds like Fidelity Spartan 500 Index (FUSEX) where the expense ratio is 0.10% and not in Vanguard S&P500 ETF (VOO) whose expense ratio is lower (0.05%) despite having the same return? What are the benefits of owning a vanguard 500 index fund or a vanguard total stock index fund? What are the obligations when purchased?