In fact, it is a relationship of trusteeship and supervision. For example, A asks B to help buy and sell stocks to earn a share. A wants to give the money directly to B, and he is afraid that B will take the money and run away. So I put the money in the bank, and agreed with the bank that B could not touch the money, but only send orders for buying and selling, and the money could only be deposited and withdrawn by A. The bank still had to watch A, and B could not be allowed to mess around. That's it.

The same is true for securities companies.