Sometimes it's better when someone who has a financial interest in a person pays for it, and not the person themselves. While Hamilton can afford his own accountability partner, it's a different situation when a outside source pays for someone to stay with him on the road, eat dinner with him, manage his money, etc. And since it's from an outside source, while Hamilton can eventually veto him, the consequences of a veto will go back to the outside source. Chances of a veto wouldn't be very common.
If Hamilton is paying, then it would be easier to veto the services, then do what they want. Because Hamilton is the boss.