You are a manager of a technology company, and your engineers love Git in general, but Git is not a perfect fit to your organization. Perhaps some work-flow elements your people are used to are not supported nicely by today's Git. Perhaps some class of assets you want to keep track of are not supported well by today's Git.
You are wealthy enough to pay for a developer or two to identify, design and implement necessary changes to Git, but you are not wealthy enough fork Git to maintain such a change yourself forever while the upstream Open Source community continues to improve Git.
What can you do?
Of course, if the changes you initially develop are good enough, they may be merged to the upstream and then you do not have to worry about maintaining your fork yourself. But how would you ensure that the quality of your changes is good enough for upstreaming? Perhaps withhold the payment to your consultants until the changes hit the upstream?
I do not think this is necessarily limited to Git, but applies equally to any useful and active Open Source project.