Why is it important to stop the germans from accessing the oil fields of saudi arabia?

The oil fields would enable Germany to fuel their air forces, making them harder to defeat.

If you are talking about WW2. Germany never had access to fuel except from export. Since they were at war now, Britain had blockaded all access to oil for Germany, and their production tanked. Germany had two options for oil, the middle east and the Caucus region in the Soviet Union. It was important to cut off this supply to Germany because not only would it limit their use of aircraft and tanks. But it would also slow supply efforts and troop movements which would otherwise cost precious oil. These 2 aspects were essential to the fast pincer movements that Germany needed to go on the offensive.