Its all about traction and track layout. The more traction you have the less ackermann you want. Too much ackerman causes the inside wheel to go past its optimum slip angle and drag against the ground. In low traction on tight corners this can help the car slow down and turn into a corner.
On high bite or open layout tracks too much ackermann can hurt your corner speed and run time.
The ackerman rate adjustment lets you split the diffrence between two ackerman settings. You can have ackermann present in a linear fashion throughout the wheels travel or you can set it so the ackerman angle diffrence between the wheels is reduced as the wheel approaches full lock.
This adjustment is for fine tuning after a good general setup has been found.
If you want to race and have fun you dont have to fool around with any of this. The kit parts and settings are great most of the time. This is the beauty of the Mi3. You can just race it or if you really want to go for it you can make small isolated changes to really dial in your car.