Being able to make a sale is a highly valuable skill. Salesmen at all levels are well-paid. The typical compensation model is a percentage of the deal, so if you manage to sell someone a large company, you’re going to make a lot of money.
Are they “overpaid”? Who is to say? The sellers of the company are the ones doing the hiring, and they do so pretty consistently. It is the job of the investment banker to maximize the sales price for the seller. They do this by doing what the seller almost never is able to do themselves - identify potential buyers, market the story, negotiate the deal, and more.
In my company, I’ve been involved in both buying and selling. We almost always have to deal with the bankers when buying and, for us, they bring order and predictability to the process. Often, they can be a voice of reason back to the seller, when said seller thinks their company is worth more than it really is.
Useful role? Yes. Overpaid? Not when the payment is voluntarily agreed by the seller.