Hourly billing works against you in the negotiation process.
Think about it. If you hire a good, competent attorney, or consultant, to help you negotiate an employment agreement or big deal, the hourly rate is likely at least $400-$500. That's the entry level. It goes up from there.
What thoughts are going through your head when you are the one personally paying that hourly rate?
Most people are likely thinking, "Hurry up! Let's get this done. The meter is running!"
This works against you in negotiations. The side of the table that owns the game clock usually wins. Good negotiations take time. The process might need to go back and forth several times with proposals and counter offers.
If you build a running meter in your own head that is pushing your emotional hot buttons to short circuit the process it's creating a problem for you. It might also cause you to choose an attorney, or consultant, who is less qualified because you are trying to save money. The hundreds of dollars you are trying to save will cost you many thousands of dollars from bad outcomes.
A value based model helps ensure you get the expertise you need and control the game clock better at the negotiating table. Pay at the end when the deal is done.