It's really not as simple as carbs are either used for energy or they turn into fat. Lyle McDonald has a great article that talks about Nutrient Storage and Oxidation.
Carbohydrates can be stored as liver or muscle glycogen, under rare
circumstances they are converted to and stored as fat. Dietary fat is
stored either in fat cells or can be stored within muscle as
intra-muscular triglyceride (IMTG). Under certain pathological
conditions, fat gets stored in places it’s not supposed to go, a
situation called ectopic fat storage. In a very real sense there’s no
true store of dietary protein although amino acids from protein
digestion are used to make various proteins and hormones in the body.
Skeletal muscle is, in essence, a ‘store’ of protein in the body.
There is no store of alcohol in the body.
But for the most part, ingested dietary fat has little impact on fat
burning in the body; that is, when you eat dietary fat, your body
doesn’t increase fat oxidation. One exception is if an absolutely
massive amount of fat (like 80 g) is consumed all at once but even
then the effect is fairly mild. Some specific fats, notably medium
chain triglycerides, are somewhat of an exception to this; they are
oxidized in the liver directly. Rather, the primary controller of
dietary fat oxidation in the body is how many carbohydrates you’re
eating, which I’ll explain momentarily.
[Since the body only has around ~500 grams of carb stores it]is
extremely good at modulating carbohydrate oxidation to carbohydrate
intake. Eat more carbs and you burn more carbs (you also store more
glycogen); eat less carbs and you burn less carbs (and glycogen levels
drop). This occurs for a variety of reasons including changing
insulin levels (fructose, for example, since it doesn’t raise insulin,
doesn’t increase carbohydrate oxidation) and simple substrate
availability. And, as it turns out, fat oxidation is basically
inversely related to carbohydrate oxidation.
As I mentioned above, an under-appreciated fact is that about half of
all ingested dietary protein is metabolized in the liver (details on
this can be found in The Protein Book). Some of it is oxidized for
energy while others are converted into other things (including glucose
and ketones) for use elsewhere. But, protein oxidation rates do
change in response to intake. So, when protein intake goes up,
oxidation will increase; when protein intake goes down, oxidation
rates decrease. This change isn’t immediate (as it more or less is
for carbohydrates) and takes 3-9 days to occur but mis-understanding
of this process has led to some goofy ideas such as protein cycling.
Having gotten that out of the way, we can start to understand the role of fat from A Primer on Nutrition pt 2
Of course, a primary role of dietary fats in the body is to be used
for energy and it was assumed for many years that this was the only
real role of fat, to provide energy storage. This was especially true
of stored body fat which was thought for decades to provide only a
passive storage depot of energy; rather it turns out that fat cells do
much more in the body, producing hormones and such that affect myriad
processes elsewhere in the body (a topic I’ve discussed at length on
the site and in my books).. Fats are also found in the cell membranes
of various tissues (and the type of fat stored there can affect
various cellular processes). As well, fats can be used to make
eicosanoids, chemical messengers made from specific fatty acids that
affect numerous biological processes. Specific dietary fats can also
affect gene expression in certain cells, impacting on things like fat
storage and oxidation and many others.
So what is dietary fat used for before turning into body fat?
Fat is mainly an energy source and the body's favorite one to store. It also has a host of other known and unknown functions in the body as per the quote above. Fat is also the only source of essential fatty acids, fat soluble vitamins, and other nutrients.
I learned from a previous question here that dietary cholesterol
doesn't necessary increase cholesterol in the bloodstream. Maybe the
same principle applies to fat...
As always, the primary determinant of whether stored fat stays stored or gets used for energy is overall caloric balance. If you eat caloric excess, you will store fat. From carbs and proteins, this will occur once lean storage depots are full or inaccessible. Although fats are stored very efficiently, those stores will be tapped for energy (for example during sleep) unless you remain in an overall caloric surplus for the day.