Same reason NYSEARCA:GLD and others exist, so you don't have to worry about securing the underlying (your broker/trading platform takes care of it), you can short it easier, etc.
Being able to buy BTC-ETF would be nice within your 401k/Roth IRA/etc.
(It would also be awesome to get a 401k to offer BTC, even if it has to be within a "self directed" fund. I think I know someone on hn who has family members in the 401k administrator business...)
