The simplest answer is that alot of people would like to hold BTC in a tax advantaged account and an ETF wrapper is the easiest way to do this.

Others would like to buy it via retirement plans and having an ETF makes this doable.

Those that want to hold BTC incase the US dollar collapses will hodl their own BTC.

Those that want to hold BTC incase it really appreciates will hold it via an ETF in a tax advantaged account so they don't have to pay taxes on their gains, depending on the account type.