Cars and tracks are living things. They change with temperature, weather, usage and many other factors so that perfect setup you nailed in testing yesterday might not feel so perfect today. Also, your car is going to change alot during the race, particularly if its a longer stint: fuel weight will drop, tyres will change pressure and their grip level will tail off, brakes will heat and wear. All this means you need to be able to recognise these changes and adapt your style to drive around them.
You will also need to adapt when you get in a new car, quickly figure out the control weight, feedback, handling balance and style the car needs. Every lap you need to be giving the maximum so you need to get up to speed as soon as possible.
Below is a video from living legend Alex Zanardi showing how well he has adapted to the new hand controls in his BMW M8 GTE during testing at Daytona.