Normally, private placement programs are nothing more than a pre-arranged buy/sell transaction of discounted banking instruments using an arbitrage transaction.
If an investor had funds of say $100M to $500M, they could create their own trading program by creating for themselves the buy/sell transaction. That is not as simple as it sounds since he would have establish control of the whole process by making contact with the Provider banks for the bank instruments and at the same time for the exit buyers.
With all the FED restrictions they have to meet this is not a simple task, plus it is no easy feat to develop the strong necessary connections with the related parties in private placement programs (the issuing banks/providers for the bank instruments and the exit-buyers).
For an investor it is much simpler and usually more profitable to enter a program where the Trader with his Trading Group has already everything in place, such as the issuing banks, the exit buyers, the contracts ready for the arbitrage transaction, the line of credit with the trading banks and all of the necessary guarantees/safety for the investor, etc.) . This way, the investor needs only to agree with the contract proposed by the Trader forgetting about any other underlying problem.
Another advantage for the investor/client is that he can enter a private placement program with a substantially lower amount of money against the case to proceed by himself because he will take indirectly advantage of the line of credit of the Trading Group.