Carbs do have a purpose, of course. They are just a source of energy and by themselves completely neutral. There are only two problems with them: the way we eat them and the way our body uses them.
In nature, carbs are never found isolated. They always come with dietary fiber and in varying degrees of complexity. But in processed food, carbs are artificially designed to be as simple as possible.
From bread and pastry to candy and fruit, the food we eat has been scientifically engineered to contain the lowest common denominator. Food companies see nothing wrong with this: they want every Fig Newton to be the exact same, but this ultimately means depriving us of variety of tastes and nutrients that naturally should exist in food. This hurts us in the long run tremendously.
Even the fruit we eat and which is arguably the source of healthiest carbs we can find has no resemblance to what is found in nature. Bananas are a fine example of this.
The variety of bananas commercially available in the world has been reduced to a single variety: Cavendish. From Ecuador to Estonia, you can walk into any store and find the exact same bananas, right down to shape, size and color.
If you open those bananas up and take a bite, you will notice they have no seeds. How do they reproduce then? They don't: they are sterile, genetically engineered clones of one another. There are enormous banana plantations all around the world, and they are all growing the exact same clone of the exact same plant.
Have you ever heard of a term "banana republic" and chuckled a bit? The term has a grave history, as it denotes a country where the banana cartel (such a thing does exist) established a puppet government and shifted the entire economy towards a single commodity: bananas. But, as it turns out, this is completely unsustainable in the long run.
The problem with growing genetic clones of any plant on a global scale is that an ecosystem is only as healthy as it is diverse. Once you create uniformity for the sake of marketability, the entire structure is vulnerable to a single agent of catastrophic failure. In this case, the culprit has a perfectly fitting ominous name: Fusarium oxysporum aka Tropical Race 4.
The description of this fungus reads like the description of the Terminator: it is indestructible, has no mercy and never stops. When this fungus that's been honing its banana-slaying skills for millennia reaches the sterile, cloned Cavendish plantations, the effect is utterly devastating. Every single plant gets infected, wilts and then the fungus lies in wait in the soil for decades, rendering it toxic to bananas. For now, the majority of banana plantations are safe while customers remain blissfully unaware of the entire fracas.
Now to consider carbs as a source of energy. Carbs are like rocket fuel for the body. But, if they are taken into the body but aren't actually used, they start causing problems.
I do want to point out the abundance of fat in the modern diet. It is needed in minute quantities in our diet, and yet we're swimming in it. Ideal diet for a bodybuilder or an athlete would have 50% carbs, 45% protein and 5% or less fat.