What are the reasons for sauteeing with fat?
When you're sauteing something, you want it to heat up quickly, so you put it in contact with a hot piece of metal. However, the food is not perfectly flat, so there will be a few contact spots and lots of pockets of air in between. This air acts as a very good insulator, so you will end up with a piece of food that is very unevenly heated.

If, on the other hand, you add some fat, this will 'fill in the cracks' and provide a more even heating of the foodstuff. (Fat does not conduct heat as well as metal, but much, much better than air.)

Also - fat tastes good.