Well, you need to eat something for energy, and that's going to be either fat or carbohydrate. Both are good and useful energy sources, so to some extent you can just take your pick. But that said, carbs become an issue once you go over a certain amount. If you can use them (say, to replenish depleted glycogen stores) then great, but if you consume them beyond this level then they can cause insulin resistance over time, inflammation and protein glycation (both from chronically elevated blood sugar), and all of the various conditions associated with these.
So if nothing else, fats are at least a source of energy that, by including in our diets, reduce the amount of carbs we consume to non-harmful levels.
But fats are also very good for you. They are more than just an energy source, they are actual structural components of your body (this is not true of carbs, which are only an energy source and nothing more). Fats are the primary components of cell membranes (every single one of your 50 trillion cells), as well as hormones, and probably a lot more things that I can't name (I'm no biologist).
In fact, fats are so important that you're going to get them no matter what. If you eat nothing but carbs, your body will turn these into fat via processing in the liver. Unfortunately, the fats your body makes from excessive carb consumption are not as good for you as the variety of healthy fats you could be ingesting. When your body makes fat from carbs it exclusively makes palmitic acid (not necessarily a "bad" fat, but certainly not the only one you want to get), and your body tends to package this fat in VLDL molecules (bad cholesterol). By contrast, the healthy fats you get from a primal diet result in a much healthier cholesterol profile.