<prompt> Human: Hey everyone,Im just having trouble grasping something. If ETFs are so popular, and the average MER on an ETF is much lower than the MER on a mutual fund, why are mutual funds so widely used?Thanks  <response>: ETF lets you trade at intra day prices. Realistically, nobody who is reading this should be trading their long term diversified retirement funds at intraday prices.I like to be able to add any amount I want, like a round 100. With ETFs you are forced to buy in multiples of the share value.Many ETFs have nasty BidAsk spreads that are another fee for the investor. The most popular ETFs dont, but the niche ones often do. So a nuanced portfolio with tilts to certain sectors or regions should use funds instead of ETFs for their more exotic holdings because BidAsk spreads are often very high on these kinds of ETFs.ETFs are great tool and do have their uses. You are not obligated to use them though. Some people do crazy things like selling put options on SPY to get steady income, but risk losing a lot if the market crashes. Im not recommending any strategy, just the opposite, do your research first. 