We aim to gather the most accurate info we can from the DiL and computer simulations, in order to be in the best shape possible, come FP1.
But of course, these simulations can never be 100% accurate and are only as good as the data that's put into them. Because of this, simulations are never used to finalise decisions, but instead used to assist and influence the direction that the engineers go down.
Once the path has been chosen, it is then down to the drivers to provide real-world feedback and for the engineers to take this information onto the next level. The full car model can never be perfectly accurate, and you also can't model the grip of the tarmac and how the tyre responds to it.
Some aspects of the car set-up, such as wing levels, are easier to gather information on through simulations while others, such as car balance or grip levels, are more difficult because they depend on more factors - some of which are outside your control, such as the weather.
We can make assumptions for the simulations, but you'll only really know how all of the elements interact together once the car is out on track for real.
The important thing for the race engineering department is to understand how to interpret that simulation information and, given its limitations, how to combine it with the real-world data, bringing it together to make the right decisions.