Thanks for clarifying Fi5h. They are an excellent questions!
Question 1: Why is there oil in the curry base?
a) Most, if not all, British Indian Restaurant observations suggest that they add oil to their curry bases (irrespective of whether or not they fry their onions, spices, etc, to start with...and I am uncertain as to what percentage of them do this...but my feeling is that the majority don't...I feel that they simply bung all the ingredients in and heat them)
b) I'm not sure of the exact science, but the oil is the solvent for the "essential oils") in the spices. The "essential oils" are insoluble in water. Therefore, oil is used to extract the flavour (i.e. the "essential oils") from the spices.
Question 2: How would the presence of oil in the base affect a subsequent korma (or other dish) made from it?
Following from my answer above, I suggest that it will result in a more flavoursome korma (or any other dish) because the oil will have extracted the flavour (i.e. the "essential oils" from the spices in the base.
Others may have alternative responses to your questions?