Bitcoin ETF, as I understand is a way of indirectly buying bitcoin without needing to go through all the processes associated with purchase (exchanges, wallets, private keys etc)
It's basically a share of bitcoin which can be tradeable in external exchanges and would be classified as a security and would mirror the value of actual bitcoin.

It has been presented as a door to bring institutional investors into bitcoin and increase the adoption and price.

But I've come across articles where institutional investors are said to be buying bitcoin through OTC trades.

My question is;
Why would investors wait to buy bitcoin indirectly, when they can buy now?

And is the grounds of possible manipulations on which the proposals has been rejected a solid argument?

And lastly would bitcoin ETF approval herald the bull run as most people believe?