Why do many recipes instruct you to oil or grease the pan at the start of the recipe?
There are three good reasons to oil your pan beforehand:


Your pan needs to be ready as soon as your batter is.  For cakes, the batter can fall apart while you are greasing the pan.  This is especially true for cakes that have air whipped in, such as genoise, which can fall apart in a few minutes.  Recipes containing baking soda can lose their fizz in this time too.
You can't forget to oil up the pan.  In the heat of the moment, it's very easy to  forget to apply lubrication before transferring the batter/dough; inevitably this wrecks the recipe.
Greasing the pan is part of baking mise en place and should be done before main cooking.  Mise en place is a key principle of professional cooking, and means "everything in place." The idea is that ingredients should be prepped and ovens preheated, so you can do the actual cooking efficiently.