Think about this scenario for a minute: You are top designer at an F1 team. You've got an idea that will (maybe) make the car go faster. Say it is a different profile rear wing. So you get the lads to build one up (you have your own carbon fibre maufacturing facility on site) and you mount it on the current car and then arrange for the team to rent (say) Brands Hatch for the day, run the new wing, compare sector time…
You might not like it, but it certainly does reduce expenses.