Insulin is elevated when your blood sugar is elevated (which happens when you ingest simple carbs & sugars). When that happens, your body goes from it's fat burning mode (default state) to carb burning. Basically, insulin is elevated and your body starts storing the glucose as glycogen in your muscles. When those stores are full, then it stores the rest as fat.

Maintaining normalized insulin levels keeps your body burning fat, which is a slower burning, preferred source of energy, but most people eat so many carbs that their body never relearns how to do it.

This isn't rocket science, it's basic biology.